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Monographs  on  Experimf.ntm  biology 

f 

EDITED  BY 

JACQUES  LOEB,  Rockefeller  Institute 

T.  H.  MORGAN,  Columbia  University 

W.  J.  V.  OSTERHOUT,  Harvard  University 


THE  PHYSICAL  BASIS  OF  HEREDITY 

BY 

THOMAS  HUNT  MORGAN 


MONOGRAPHS  ON  EXPERIMENTAL 

BIOLOGY 


PUBLISHED 

VOLUME  I 

FORCED  MOVEMENTS,  TROPISMS,  AND 

ANIMAL  CONDUCT 

By  JACQUES  LOEB,  Rockefeller  Institute 
IN  PREPARATION 

THE  CHROMOSOME  THEORY  OF 
HEREDITY 

By  T.  H.  MORGAN,  Columbia  University 

INBREEDING  AND  OUTBREEDING:  THEIR 

GENETIC  AND  SOCIOLOGICAL 

SIGNIFICANCE 

By  E.  M.  EAST  and  D.  F.  JONES,  Bussey   Institution, 

Harvard   University 

PURE  LINE  INHERITANCE 

By  H.  S.  JENNINGS,  Johns  Hopkins  University 

THE   EXPERIMENTAL  MODIFICATION  OF 
THE  PROCESS  OF  INHERITANCE 

By  R.  PEARL,  Johns  Hopkins  University 

LOCALIZATION  OF  MORPHOGENETIC 
SUBSTANCES  IN  THE  EGG 

By  E.  G.  CONKLIN,  Princeton  University 

TISSUE  CULTURE 

By  R.  G.  HARRISON,  Yale  University 

PERMEABILITY  AND  ELECTRICAL 
CONDUCTIVITY  OF  LIVING  TISSUE 

By  W.  J.  V.  OSTERHOUT,  Harvard  University 

THE  EQUILIBRIUM  BETWEEN  ACIDS  AND 

BASES  IN  ORGANISM  AND 

ENVIRONMENT 

By  L.  J.  HENDERSON,  Harvard  University 

CHEMICAL  BASIS  OF  GROWTH 

By  T.  B.  ROBERTSON,  University  of  Toronto 

PRIMITIVE  NERVOUS  SYSTEM 

By  G.  H.  PARKER,  Harvard  University 

COORDINATION  IN  LOCOMOTION 

By  A.  R.  MOORE,  Rutgers  College 
OTHERS  WILL  FOLLOW 


Monographs  on  Experimental  Biology 


THE  PHYSICAL  BASIS 
OF  HEREDITY 


BY 

THOMAS  HUNT  MORGAN 

PROFESSOR    OF    EXPERIMENTAL    ZOOLOGY    IN    COLUMBIA    UNIVERSITY 


JI7  ILLUSTRATIONS 


PHILADELPHIA  AND  LONDON 
J.  B.  LIPPINCOTT  COMPANY 


COPYRIGHT,    I919.    BY  J.   B.    LIPPINXOTT  COMPANY 


Electrotyped  and  Printed  by  J.  B.  LiPPincott  Company 
The  Washington    Square    Press,   Philadelphia,    U.  S.  A. 


EDITORS'  ANNOUNCEMENT 

The  rapid  increase  of  specialization  makes  it  im- 
possible for  one  author  to  cover  satisfactorily  the  whole 
field  of  modern  Biology.  This  situation,  which  exists  in 
all  the  sciences,  has  induced  English  authors  to  issue 
series  of  monographs  in  Biochemistry,  Physiology  and 
Physics.  A  number  of  American  biologists  have  decided 
to  provide  the  same  opportunity  for  the  study  of 
Experimental  Biology. 

Biology,  which  not  long  ago  was  purely  descriptive 
and  speculative,  has  begun  to  adopt  the  methods  of  the 
exact  sciences  recognizing  that  for  permanent  progress 
not  only  experiments  are  required  but  quantitative  experi- 
ments. It  will  be  the  purpose  of  this  series  of  monographs 
to  emphasize  and  further  as  much  as  possible  this  develop- 
ment of  Biology. 

Experimental  Biology  and  General  Physiology  are  one 

and  the  same  science,  in  method  as  well  as  content,  since 

both  aim  at  explaining  life  from  the  physico-chemical 

constitution  of  living  matter.   The  series  of  monographs 

on  Experimental  Biology  will  therefore  include  the  field 

of  traditional  General  Physiology. 

Jacques  Loeb, 
^'  T.  H.  Morgan, 

r\V\  W^\  ^*  *^*  ^*  OSTERHOUT. 


lA 


CONTENTS 

CHAPTER  PAOB 

I.  Introduction 15 

11.  Mendel's  First  Law — Segregation  of  the  Genes 19 

III.  The  Mechanism  of  Segregation 39 

IV.  Mendel's  Second  Law — The  Independent  Assortment  of 

the  Genes 59 

V.  The  Mechanism  of  Assortment 73 

VI.  Linkage 80 

VII .  Crossing  Over 87 

VIII.  Crossing  Over  and  Chromosomes 96 

IX.  The  Order  of  the  Genes 118 

X.  Interference 126 

XI.  Limitation  of  the  Linkage  Groups 133 

XII.  Variation  in  Linkage 139 

XIII.  Variation  in  the  Number  of  the  Chromosomes  and  its  Re- 

lation TO  the  Totality  of  the  Genes 147 

XIV.  Sex-Chromosomes  and  Sex-linked  Inheritance 165 

XV.  Parthenogenesis  and  Pure  Lines 204 

XVI.  The  Embryological  and  Cyto logical  Evidence  that  the 

Chromosomes  are  the  Bearers  of  the  Hereditary  Units.  .  212 

XVII.  Cytoplasmic  Inheritance 219 

XVIII.  Maternal  Inheritance 227 

XIX.  The  Particulate  Theory  of  Heredity  and  the  Nature  of 

THE  Gene 234 

XX.  Mutation 247 


ILLUSTRATIONS 

Fig.  page 

1.  Cross  Between  a  Tall  and  a  Short  Race  of  Garden  Peas 20 

2.  Cross  Between  White  and  Red  Flowered  Four-o'clocks 24 

3.  Cross  Between  Splashed-White  and  Black,  in  Andalusian 26 

4.  Male  and  Female  Vinegar  Fly 28 

5.  Normal  and  Abnormal  Abdomen  of  D.  melanogaster 29 

6.  Relation  of  Black  Body  Color  to  Wild  Type  as  Shown  by  Classes 

of  Fhes 30 

7.  Normal,  Heterozygous,  and  Bar  Eye  of  the  Vinegar  Fly 31 

8.  Relation  of  Bar  Eye  to  Normal  Eye 31 

9.  Relation  of  Andalusian  to  Splashed  White  and  to  Black  as  Shown 

by  Classes  of  Birds 32 

10.  Relation  of  Tall  to  Short  Peas 32 

11.  Relation  of  Normal  to  Abnormal  Abdomen  as  Shown  by  Classes 

of  Flies 32 

12.  Relation  of  Normal  to  Duplicate  Legs  of  Fhes 33 

13.  Notch  Wings  in  the  Vinegar  Fly 35 

14.  Oocyte  of  Ancyracanthus;  Growth  Period;  Nucleus  with  Tetrads. . .  40 

15.  Egg  of  Ancyracanthus 40 

16.  Eggs  of  Ancyracanthus  within  Membrane 41 

17.  Spermatogenesis  of  Ancyracanthus 42 

18.  Last  Spermatogonial  Division  of  Tomopteris  and  Stages  Before  and 

During  Synapsis 45 

19.  Thin-Thread  Stage  of  Tomopteris  Spermatocyte;  Tetrads,  and  First 

and  Second  Spermatocyte  Divisions 47 

20.  Synaptic  Stages  and  Those  Immediately  Following  in  Batracoseps ...  48 

21.  Synaptic  Stages  and  Those  Immediately   Following  in  the  Egg  of 

Pristiurvs 50 

22.  Sister  Blastomeres  of  Ascaris  Preparatory  to  Another  Division ....  52 

23.  Normal  and  Reduced  Chromosomes  of  Biston 53 

24.  Division  Figures  in  Egg  of  Ctenolabrus  FertiUzed  by  Fundulus 54 

25.  Female  and  Male  Chromosome  Groups  of  Protenor 55 

26.  Reduced  Chromosome  Group;  and  Extrusion  of  Polar  Bodies  in 

Protenor "^^ 

9 


10  ILLUSTRATIONS 

27.  Reduced    Chromosome    Group  of    Male;   and  Spermatogenesis  in 

Protenor ; 56 

28.  Diploid  and  Haploid  Chromosome  Groups  of  Drosophila  busckii  and 

D.  melanica  (neglecta) 57 

29.  Cross  Between  Wingless  and  Ebony  Vinegar  Fly 65 

30.  Miniature  Wing,  Dumpy,  and  Miniature  Dumpy 66 

31.  Combs  of  Fowls 69 

32.  Eight    Chromosome    Groups    of    Twelve    Chromosomes    Each    of 

Trimeroiropis 77 

33.  Back-cross  of  Male   (Out  of  Black  Vestigial  by  Wild)    to  Black 

Vestigial 81 

34.  Back-cross  of  Male   (Out  of  Gray  Vestigial  by  Black)   to  Black 

Vestigial 83 

35.  Scheme     Showing     the     Inheritance    of     the     X-Chromosome    in 

Drosophila 84 

36.  Back-cross  of  Female  (Out  of  Black  Vestigial   by  Wild)  to  Black 

Vestigial  Male 89 

37.  Back-cross  of  Female  (Out  of  Gray  Vestigial  by  Black)  to  Black 

Vestigial  Male 90 

38.  Scheme  to  Illustrate  Double  Crossing  Over  Between  White  and 

Forked 93 

39.  Curve  Showing  the  Influence  of  Temperature  on  Crossing  Over  Control  98 

40.  Curve  Showing  the  Influence  of  Temperature  on  Crossing  Over ....  98 

41 .  Diagram  Showing  Crossing  Over  of  Two  Chromosomes  at  Four-strand 

Stage  and  the  Subsequent  Opening  Out  of  the  Tetrad 101 

42.  Scheme  Showing  the  Opening  Out  of  the  Strands  of  the  Tetrad 102 

43.  Scheme  Showing  Crossing  Over  Involving  Both  Strands  of  Each 

Chromosome 103 

44.  Spermatogonial  Cells  in  the  Last  Phase  of  Division  and  the  Following 

Resting  Stages 105 

45.  Cells  Emerging  From  the  Resting  Stages  Preparatory  for  the  Next 

Spermatogonial  Division 106 

46.  Cells  Emerging  From  Their  Last  Spermatogonial  Division 106 

47.  Formation  of  a  Thick  Thread  after  Synapsis  and   the   Following 

Condensation  of  a  Tetrad 107 

48.  A  Pair  of  Chromosomes  in  Conjugation 109 


ILLUSTRATIONS  11 

49.  The  Same  Chromosome  Pair  in  Conjugation  from  Thirteen  Different 

Cells 110 

50.  Conjugation  of  an  Unequal  Pair  of  Chromosomes  and  Their  Subse- 

quent Separation Ill 

51.  Two  Schemes  Illustrating  the  Idea  of  Reduplication  by  Bateson 

and  Punnett 116 

52.  Scheme  Illustrating  How  Double   Crossing    Over    Between    Two 

Distinct  Genes  takes  Place 121 

53.  Chromosome  Groups  of  Pea,  Wheat,  and  Primula 135 

54.  Types  of  Chromosome  Groups  Found  in  Drosophila 136 

55.  Haploid  Group  of  Chromosomes  of  the  Silkworm  Moth 137 

56.  Curve  Showing  Influence  of  Crossing  Over  at  Different  Temperatures  142 

57.  Diagram  Illustrating  the  Effect  on  Crossing  Over  Due  to  the  Presence 

of  Crossover  Grenes 143 

58.  Chromosome  Group  of  (Enothera  Lamarckiana  and  0.  gigas,  and 

Triploid  Group 149 

59.  Life  Cycle  of  Moss 152 

60.  Diagram  Illustrating  the  Formation  of  Individuals  from  the  Regener- 

ation of  the  Sporophyte  in  a  Dioecious  Species 153 

61.  Diagram  Illustrating  the  Formation  of  Individuals  from  the  Regener- 

ation of  the  Sporophyte  in  a  Hermaphroditic  Species 153 

62.  Somatic  Chromosomes  Groups  of  (Enothera  sciniillans 156 

63.  Scheme  Showing  the  Probable  Relation  Between  the  Extra  Chromo- 

some Pieces  of  Fig.  62,  and  the  Normal  Fifteen  Chromosomes  of 
This  Mutant 158 

64.  An  Egg  of  Ascaris  hivalens  Fertilized  by  Sperm  of  A.  univalens . .  .  .   160 

65.  Diploid  and  Haploid  Groups  of  the  Sundew  Drosera 160 

66.  A  Scheme  Illustrating  the  Fertilization  of  the  Egg  of  One  Species  of 

Moth  by  the  Sperm  of  Another 161 

67.  Scheme  Illustrating  the  History  of  the  Chromosomes,  and  the  Back- 

cross   Between  a  Hybrid  Male  and  One  or  the  Other  Parent ....   162 

68.  Scheme  Showing  the  Relation   of  the  Sex- Chromosome  to  Sex-De- 

termination     166 

69.  Cross  Between  White-Eyed    Male   and   a   Red-Eyed   Female  of  the 

Vinegar  Fly 168 


12  ILLUSTRATIONS 

70.  Cross  Between  White-Eyed  Female  and  a  Red-Eyed  Male  of   the 

Vinegar  Fly 169 

71.  Cross  Between  a  Yellow  White-Eyed   Female  and    a    Wild-Type 

("Gray"),  Red-Eyed  Male 171 

72.  The  Results  from  the  Reciprocal  Cross  of  That  Shown  in  Fig.  71 . . .   173 

73.  Scheme  Showing  the  Relation  of  the  Sex-Chromosomes  of  the  Moth 

in  Sex  Determination 174 

74.  Cross  Between  Abraxas  lacticolor  Female,  and  Grossulariata  Male ....  175 

75.  Cross  Between  Abraxas  grossulariata  Female  and  Lacticolor  Male.  . .  176 

76.  Cross  Between  Barred  Plymouth  Rock  Male  and  Black  Langshan 

Female 178 

77.  Scheme  Showing  the  Transmission  of  the  Sex- Linked  Characters ....   178 

78.  Cross  Between  Black  Langshan  Male  and  Barred  Plymouth  Rock 

Female 178 

79.  Scheme  Showing  the  Transmission  of  the  Sex-Linked  Characters 

Shown  in  Fig.  78 - 179 

80.  First  and  Second  Spermatocyte  Divisions  in  the  Bee 181 

81.  First  and  Second  Spermatocyte  Divisions  in  the  Hornet 182 

82.  Life  Cycle  of  Phylloxera  caryoecaulis 182 

83.  Extrusion  of  the  Polar  Body  from  a  Male-Producing  Egg 183 

84.  First  and  Second  Spermatocyte  Divisions  in  the  Bearberry  Aphid ....   184 

85.  Hydatina  senta:  Adult  Female,  Young  Female  Soon  After  Hatching, 

Adult  Male,  Parthenogenetic  Egg,  Male-Producing  Egg,  Resting 
Egg 186 

86.  Diagram  Showing  How  a  Continuous  Diet  of  Polytoma  Through 

Twenty-Two  Months  Yielded  Only  Female-Producing  Females. . .    187 

87.  A,  Gynandromorph  of  Drosophila  melanogaster ,  that  was  Female  on 

Right  Side  and  Male  on  the  Left;      B,  Female    on  the    Left  Side 
and  Male  on  the  Right 190 

88.  Diagram  Showing  Ehmination  of  X'  at  an  Early  Cell  Division 191 

89.  Caterpillars  of  the  Silkworm  Moth 192 

90.  Diagram  Illustrating  How    a  Heterozygous  Egg  With  Two  Nuclei 

Fertilized  by  Two  Sperms  Might  Produce  a  Gynandromorph  Uke 
that  Shown  in  Fig.  89 193 

91.  Scheme  Showing  the  Transmission  of  a  Lethal  Sex-Linked  Factor 

in  an  X-Chromosome 199 

92.  Normal  Female  and  Male  Groups  of  Chromosome  of  the  Vinegar 

Fly 200 


ILLUSTRATIONS  13 

93.  Non-Disjunction.     Egg  Fertilized  by  X-Sperm 201 

94.  Non-Disjunction.     Egg  Fertilized  by  Y-Sperm 202 

95.  A  Wingless  Aphid  and  a  Winged  One 207 

96.  Curve  Showing  the  Non-efifect  of  Selection  for  the  First  Twelve 

Generations  for  Increase  in  Body  Length 208 

97.  Curve  Showing  the  Effect  of  Selection  for  the  Second  Score  of 

Generations 209 

98.  Scheme  Showing  Dispermic  Fertilization  of  the  Egg  of  the  Sea 

Urchin 214 

99.  First  Division  of  a  Hybrid  Egg 215 

100.  Fertilization  of  an  Egg  Starting  to  Develop  Parthenogenetically . . .  216 

101.  Larval  Sea  Urchin  Seen  in  Side  View 217 

102.  Green  Leaf  and  Checkered  Leaf  of  Four-o'clock 220 

103.  Pelargonium  that  Gave  Rise  to  a  White  Branch 221 

104.  Diagram  to  Show  How  a  Sectorial  Chimera  May  be  Produced 221 

105.  Diagram  to  Illustrate  Maternal  Inheritance 228 

106.  Diagram  to  Show  the  Inheritance  of  Two  Pairs  of  Mendelian 

Characters 238 

107.  A,  Hen-Feathered  Campine  Male; B,  Adult  Castrated  Campine  Male ; 

C,  Sebright  Hen-Feathered  Male,  D,  Adult  Castrated  Sebright  Male  246 

108.  Diagram  Illustrating  Mutation  in  a  Nest  of  Genes 252 

109.  Two  Flies  (Drosophila)  with  Beaded  Wings 258 

110.  Diagram  Showing  the  Relation  of  the  Chromosomes 258 

111.  Diagram  to  Show  how  the  Appearance  of  a  Lethal  Near  Beaded 

Causes  the  Stock  to  Produce  only  Beaded 259 

112.  Diagram  Showing  the  Results  of  Crossing  Over  in  a  Stock  Contain- 

ing Both  Beaded  and  Lethal 260 

113.  Diagram  Illustrating  How  in  the  Presence  of  a  Dominant  Factor, 

Dichete,  and  a  Lethal  in  Its  Homologous  Chromosome  at  About 
the  Same  Level,  Together  with  Another  Factor,  Peach  Colored 
Eyes,  Gives  the  Result  Shown  in  the  Squares 261 

114.  Diagram  Illustrating  Crossing  Over  of  Factors  in  Fig.  113 262 

115.  Rosettes  of  the  Twin  Hybrids  of  the  Evening  Primrose 263 

116.  Diagram  Illustrating  Balanced  Lethals  and  Twin  Hybrids 264 

117.  Diagram  Illustrating  Lethals  and  Four  Types 265 


"I 


THE   PHYSICAL  BASIS  OF 

HEREDITY 


CHAPTER    I 

INTRODUCTION 

That  the  fundamental  aspects  of  heredity  should  have 
turned  out  to  be  so  extraordinarily  simple  supports  us  in 
the  hope  that  nature  may,  after  all,  be  entirely  approach- 
able. Her  much-advertised  inscrutability  has  once  more 
been  found  to  be  an  illusion  due  to  our  ignorance.  This 
is  encouraging,  for,  if  the  world  in  which  we  live  were  as 
complicated  as  some  of  our  friends  would  have  us  believe 
we  might  well  despair  that  biology  could  ever  become  an 
exact  science.  Personally  I  have  no  sympathy  with  the 
statement  that  *^the  problem  of  the  method  of  evolution 
is  one  which  the  biologist  finds  it  impossible  to  leave  alone, 
although  the  longer  he  works  at  it,  the  farther  its  solution 
fades  into  the  distance.''  On  the  contrary,  the  evidence 
of  recent  years  and  the  methods  by  means  of  which  this 
evidence  is  obtained  have  already  in  a  reasonably  short 
time  brought  us  nearer  to  a  solution  of  some  of  the  import- 
ant problems  of  evolution  than  seemed  possible  only  a  few 
years  ago.  That  new  problems  and  developments  have 
arisen  in  the  course  of  the  work — as  they  are  bound  to 
do  in  any  progressive  science,  as  they  do  in  chemistry  and 
in  physics  for  example — goes  without  saying,  but  only  a 
spirit  of  obscurantism  could  pretend  that  progress  of  this 
kind  means  that  we  see  the  solution  of  our  problem  fading 
away  into  the  distance. 

Mendel  left  his  conclusions  in  the  form  of  two  general 
laws  that  may  be  called  the  law  of  segregation  and  the 

16 


fROPERTY  IIBURY 

n.  C.  StaU  College 


16  PHYSICAL  BASIS  OF  HEEEDITY 

law  of  independent  assortment  of  the  genes.  They  rest 
on  numerical  data,  and  are  therefore  quantitative  and  can 
be  turned  into  mathematical  form  wherever  it  seems  desir- 
able. But  though  the  statements  were  exact,  they  were 
left  without  any  suggestion  as  to  how  the  processes 
involved  take  place  in  the  living  organism.  Even  a  purely 
mathematical  formulation  of  the  principles  of  segregation 
and  of  free  assortment  would  hardly  satisfy  the  botanist 
and  zoologist  for  long.  Inevitably  search  would  be  made 
for  the  place,  the  time,  and  the  means  by  which  segre- 
gation and  assortment  take  place,  and  attempts  would 
sooner  or  later  be  made  to  correlate  these  processes  with 
the  remarkable  and  unique  changes  that  take  place  in  the 
germ-cells.  Sutton,  in  1902,  was  the  first  to  point  out 
clearly  how  the  chromosomal  mechanism,  then  known, 
supplied  the  necessary  mechanism  to  account  for  Mendel's 
two  laws. 

The  knowledge  to  which  Sutton  appealed,  had  been 
accumulating  between  the  years  1865,  when  MendePs 
work  was  published,  and  1900,  when  its  importance  became 
generally  known.  An  account  of  the  chromosomal 
mechanism  may  be  deferred,  but  I  have  spoken  of  it  here 
in  order  to  call  attention  to  a  point  rarely  appreciated, 
namely,  that  the  acceptance  of  this  mechanism  at  once 
leads  to  the  logical  conclusion  that  MendePs  discovery 
of  segregation  applies  not  only  to  hybrids,  but  also  to 
normal  processes  that  are  taking  place  at  all  times  in  all 
animals  and  plants,  whether  hybrids  or  not.  In  conse- 
quence we  find  that  we  are  dealing  with  a  principle  that 
concerns  the  actual  composition  of  the  material  that  car- 
ries one  generation  over  to  the  next. 

Segregation  and  independent  assortment  were  the  two 
fundamental  principles  of  heredity  discovered  by  Mendel. 
Since  1900,  four  other  principles  have  been  added.  These 
are  known  as  linkage,  the  linear  order  of  the  genes,  inter- 
ference, and  the  limitation  of  the  linkage  groups.  In  the 
same  sense  in  which  in  the  physical  sciences  it  is  custo- 


INTRODUCTION  17 

mary  to  call  tlie  fundamental  generalizations  of  the  science 
the  ''laws"  of  that  science,  so  we  may  call  the  foregoing 
generalizations,  the  six  laws  of  heredity  known  to  us  at 
present.  Despite  the  fact  that  the  use  of  this  word  '4aw" 
has  been  much  abused  in  popular  biological  writing  we 
need  not  apologize  for  using  it  here,  because  the  postu- 
lates in  question  have  been  established  by  the  same  scien- 
tific procedure  that  chemists  and  physicists  make  use  of, 
viz.,  by  deductions  from  quantitative  data.  Excepting  for 
the  sixth  law  they  can  be  stated  independently  of  the  chro- 
mosomal mechanism,  but  on  the  other  hand  they  are  also 
the  necessary  outcome  of  that  mechanism. 

The  theory  of  the  constitution  of  the  germ-plasm, 
to  which  Mendel's  discoveries  led  him,  not  only  failed  to 
receive  any  recognition  for  fifty  years,  but  the  principle 
of  particulate  inheritance  to  which  it  appeals  has  met 
with  a  curious  reception  even  in  our  own  time,  leading 
a  recent  writer  to  state  that  particulate  theories  in  general 
"do  not  help  us  in  any  way  to  solve  any  of  the  funda- 
mental problems  of  biology, ' '  and  another  writer  to  affirm 
that  if  the  chromatin  of  the  sperm  is  "pictured"  as  com- 
posed of  individual  units  that  represent  "some  specific 
unit-characters  of  the  adult, ' '  then  we  should  expect  it  to 
be  extremely  complex,  "more  complex  indeed  than  any 
chromatin  in  the  body,  since  it  is  supposed  to  represent 
them  all,"  but  "as  a  matter  of  fact  chemical  examination 
shows  the  chromatin  in  the  fish  sperm  to  be  the  simplest 
found  anywhere. ' '  Were  our  knowledge  of  the  chemistry 
of  the  "chromatin"  as  advanced  as  these  very  positive 
statements  might  lead  one  to  suppose,  the  objection  raised 
might  appear  to  be  serious,  but  there  is  no  evidence  in 
favor  of  the  statement  that  the  sperm-chromatin  should  be 
expected  to  be  more  complex  than  the  same  chromatin 
in  the  cells  of  the  embryo  or  adult.  And  even  were  it 
different  in  the  germ-tract  and  soma  the  criticism  would 
miss  its  mark,  because  heredity  deals  with  the  constitution 
of  the  chromatin  of  the  germ-tract  and  not  with  that  of 

2 


18  PHYSICAL  BASIS  OF  HEEEDITY 

the  soma.  Until  physiological  chemists  are  in  position 
to  furnish  more  complete  information  concerning  the  com- 
position of  the  chromosomes,  or  more  illuminating  criti- 
cism of  the  situation  as  it  exists,  we  need  not,  I  think, 
be  over-much  troubled  by  such  views  so  long  as  we  handle 
our  own  data  in  a  manner  consonant  with  the  recognized 
methods  of  scientific  procedure. 

Other  critics  object  for  one  reason  or  another  to  all 
attempts  to  treat  the  problem  of  heredity  from  the  stand- 
point of  the  factorial  hypothesis.  It  has  been  said,  for 
instance,  that  since  the  postulated  genetic  factors  are 
not  known  chemical  substances  the  assumption  that  they 
are  such  bodies  is  presumptuous,  and  gives  a  false  analogy 
with  chemical  processes.  Such  critics  claim  that  the  pro- 
cedure is  at  best  only  a  kind  of  symbolism.  Again,  it  has 
been  said,  that  the  factorial  hypothesis  is  not  a  real 
scientific  hypothesis,  for  it  merely  restates  its  facts  in 
terms  of  factors,  and  then  by  juggling  with  numbers  pre- 
tends that  something  is  being  explained.  It  has  been 
argued  that  Mendelian  phenomena  relate  to  unnatural 
conditions  and  that  they  have  nothing  to  do  with,  the 
normal  process  of  heredity  in  evolution  that  takes  place 
in  ''nature."  It  has  been  objected  that  such  a  hypoth- 
esis assumes  that  genetic  factors  are  fixed  and  stable  in 
the  same  sense  that  molecules  are  stable,  and  that  no  such 
hard  lines  are  to  be  found  in  the  organic  world.  And 
finally  it  has  been  urged  that  the  hypothesis  rests  on  dis- 
continuous variation  which,  it  is  said,  does  not  exist. 

If  the  implications  in  any  or  in  all  of  these  objections 
were  true,  the  attempt  to  explain  the  traditional  prob- 
lem of  heredity  by  the  factorial  hypothesis  would 
appear  fantastic  in  the  extreme.  An  attempt  will  be 
made  in  the  following  chapters  to  present  the  evidence 
on  which  our  present  views  concerning  heredity  rest,  in 
the  hope  that  an  understanding  of  this  evidence  will  go 
far  towards  removing  these  a  priori  objections,  and  will 
show  that  they  have  no  real  foundation  in  fact. 


CHAPTER   II 

MENDEL'S  FIRST  LAW— SEGREGATION 

OF  THE  GENES 

Mendel  succeeded  in  discovering  the  principle  of 
segregation  because  he  simplified  the  conditions  of  his 
experiments  so  that  he  had  to  deal  with  one  process  at 
a  time.  Others  before  him  had  failed  because  they  worked 
with  too  complex  a  situation.  In  each  case  Mendel  picked 
out  for  study  a  pair  of  contrasted  characters  of  a  kind 
that  were  sharply  distinguishable  from  each  other  when- 
ever they  appeared.  He  chose  plants  that  normally  self- 
fertilize  and  are  little  liable  to  accidental  cross-fertiliza- 
tion, which  made  it  possible  easily  to  obtain  in  the  second 
generation  numbers  large  enough  to  give  significant 
results.  To  MendePs  foresight  in  arranging  the  condi- 
tions of  his  work,  as  much  as  to  his  astuteness  in  interpret- 
ing the  data,  is  due  his  remarkable  success. 

Mendel  used  varieties  of  the  common  edible  garden 
pea  (Pisum  sativum).  Many  of  these  varieties  (races) 
differ  from  each  other  in  a  particular  character.  Some 
races  are  tall,  others  short;  some  have  green  peas  (seeds 
in  the  pods),  others  have  yellow  peas ;  some  of  these  seeds 
have  a  smooth  surface,  others  are  wrinkled;  some  of  the 
pods  are  hard,  others  are  soft.  One  of  the  crosses  made 
by  Mendel  will  serve  as  an  illustration  of  his  work  (Fig.  1). 

Pollen  from  a  race  of  tall  peas  was  put  artificially  on 
the  stigma  of  a  plant  of  a  short  race,  whose  own  stamens, 
and  therewith  the  pollen,  had  been  previously  removed. 
The  hybrid  plants  that  came  from  the  seed  were  tall. 
These  hybrids  were  allowed  to  self-fertilize  and  their 
seeds  collected.    Some  of  the  seeds  produced  tall  jolants, 

19 


20 


PHYSICAL  BASIS  OF  HEREDITY 


Tall 


@@ 


Sfiott  \s\  \s\ 


Fig.  1. — Cross  between  a  tall  and  a  short  race  of  garden  peas.  The  F\  generation  is 
tall.  In  the  second  generation,  Fi,  there  are  three  talis  to  one  short.  {P\,  F\  and  F2  were 
reared  from  peas  supplied  by  Dr.  O.  E.  White.) 


MENDEL'S  FIRST  LAW  21 

others  produced  short  plants;  in  the  ratio  of  3  tall  to  1 
short.  In  other  words,  the  contrasted  characters  of  the 
grandparents  reappeared  in  the  grandchildren  in  the  ratio 
of  3  to  1.  The  experiment  was  carried  through  one  more 
generation,  which  was  necessary  in  order  to  get  data 
for  finding  out  what  had  been  taking  place.  The  short 
peas  were  allowed  to  fertilize  themselves.  They  pro- 
duced only  short  peas.  The  tall  peas  were  also  allowed  to 
fertilize  themselves.  One-third  of  the  tall  peas  produced 
only  tall  offspring;  two-thirds  produced  both  tall  and 
short  offspring  in  the  ratio  of  3 : 1,  as  had  the  first  genera- 
tion hybrids.  Evidently  then  the  grandchildren  had  been 
of  three  kinds,  one  kind  was  pure  for  shortness,  others 
were  hybrids,  and  the  remaining  kind  was  pure  for  tall- 
ness.  These  kinds  appeared  in  the  proportion  of  1 :  2 : 1. 
Some  factor  or  factors  in  the  original  tall  peas  must 
cause  the  peas  of  that  race  to  be  always  tall,  and  some 
factor  in  the  original  short  peas  must  cause  them  to  be 
short.  The  short  factor  may  be  represented  by  s,  and 
the  long  factor  by  S.  When  crossed,  the  fertilized  egg 
should  contain  both  factors  (sS),  and  since  the  hybrids 
coming  from  this  egg  were  tall,  it  is  evident  that  tall  must 
dominate  over  short.  Now  if  the  two  factors  (sS)  present 
in  the  hybrid  should  separate  {i.e.,  "segregate'')  when  its 
ovules  and  its  pollen-grains  are  formed,  half  of  the  eggs 
would  contain  the  factor  that  represents  the  short  peas 
(5),  and  half  of  the  eggs  the  factor  that  represents  tall 
peas  (S) ;  also  half  of  the  pollen  grains  would  contain  the 
factor  that  represents  the  short  peas  (5),  and  half  of  them 
would  contain  the  factor  that  represents  the  tall  peas  (S). 
Chance  meeting  between  egg-cells  and  pollen-cells  (one 
ovule  being  always  fertilized  by  one  pollen  grain),  would, 
on  the  average,  give  one  fertilized  egg  containing  two 
factors  for  short  (55) ;  to  two  fertilized  eggs  that  contain 
one  of  each  kind  of  factor  (sS) ;  to  one  that  contains  two 


22  PHYSICAL  BASIS  OF  HEREDITY 

factors  for  tall  (SS).    The  chance  combination  just  given 
may  be  represented  graphically  as  follows: 


Tall  ,^         ^   Short 
Tall  /      \.  Short 


(Tall-Short. 
Tall-Tall.  -f^  +Sliort-Short. 

^  riall-Short. 


In  the  actual  experiment  that  Mendel  carried  out,  plants 
of  the  tall  race  measured  from  6  to  7  feet,  and  those  of 
the  short  plants  three-quarters  to  one  foot  and  a  half. 
The  F^  plants  were  as  tall  as,  or  even  taller  than  the  tall 
parent.  When  these  7^"'/s  were  self-fertilized,  the  seeds 
(either  from  the  same  plant  or  from  a  random  collection 
of  seeds  from  different  F^  plants)  produced  787  long 
plants  and  277  short  plants — a  ratio  of  2.84  to  1. 

As  a  fair  sample  of  each  plant,  ten  seeds  were  taken 
from  each  of  100  tall  plants  of  tliis  second  (or  F2)  genera- 
tion. Out  of  the  100  plants  so  tested,  28  plants  produced 
only  tall  plants,  while  72  of  them  produced  some  tall 
and  some  short  offspring.  This  means  that  28  plants 
were  pure  (homozygous)  tall,  whilst  72  were  hybrid  like 
the  F^  plants.  Taking,  then,  all  F2  plants  together,  the 
results  show  %  were  short,  V4  were  hybnd,  and  %  were 
tall,  i.e.,  they  stand  in  a  ratio  of  1 :  2 : 1. 

This  relation  is  illustrated  in  the  scheme  below,  based 
on  what  16  F2  plants  might  give.  Twelve  would  be  tall 
to  4  short.  If  the  tall  plants  are  tested,  they  are  found 
to  consist  of  4  pure  tails  {.SS)  and  8  hybrid  tails  {sS). 
Altogether,  then,  there  are  4  tails  to  8  hybrid  tails  to  4 
short,  i.e.,  there  are  three  kinds  of  F2  peas  in  the  ratio 
of  1:2:1. 

12  tall  -f  4  short 


4SS    4-  8sS  +  4ss 

1  2  1 


The  process  of  disjunction,  or  separation  of  the  mem- 
bers of  a  pair  of  factors,  is  known  technically  as  segre- 
gation.    While  we  sometimes  also  speak  of  the  segrega- 


MENDEL  ^S  FIRST  LAW  23 

tion  of  the  characters  themselves,  it  seems  better,  I  think, 
to  avoid  as  far  as  possible  this  application  of  the  word. 
The  factor  for  tall  and  the  factor  for  short  are  said  to  be 
allelomorphic  to  each  other.  The  parents  are  generally 
designated  by  P^ ;  the  first  hybrid  generation  is  known 
as  the  first  filial  generation,  or  briefly  F^.  The  next 
generation,  derived  from  F^  is  called  Fg,  etc.  When  one 
member  of  the  pair  of  contrasted  characters  appears  in 
F^  to  the  exclusion  of  the  other  it  is  said  to  be  dominant, 
the  eclipsed  character  is  said  to  be  recessive.  The  hybrid 
itself  is  said  to  be  heterozygous,  meaning  that  it  contains 
one  factor  or  gene  of  each  kind,  while  an  individual  con- 
taining both  genes  of  the  same  sort  is  said  to  be  homo- 
zygous for  the  genes  involved.  Mendel  did  not  emphasize 
the  idea  that  even  in  pure  races  each  character  is  also 
represented,  as  a  rule,  by  a  pair  of  factors  or  genes  that 
segregate  in  the  formation  of  the  germ-cells  in  the  same 
way  as  do  the  pair  of  contrasted  genes  in  the  hetero- 
zygotes,  but  at  the  present  time  this  idea  is  accepted  by 
all  geneticists.  It  was  at  least  implied  on  Mendel's  view 
that  the  two  pure  classes  in  F^  {SS  and  ss),  formed  by 
the  recombination  of  two  like  genes,  are  identical  with 
the  two  grandparental  races  (Pi). 

A  crucial  test  of  the  correctness  of  the  assumption  that 
segregation  of  the  members  of  a  pair  of  elements  takes 
place  in  the  germ-cells  of  the  hybrid,  consists  in  back- 
crossing  the  hybrid  (F^)  to  one  of  the  parent  stock,'  viz., 
to  the  not  dominant  stock,  here  the  short  pea.  Since  short 
is  recessive  to  tall,  it  will  not  influence  the  height  of  the 
offspring  when  a  tall  and  a  short  factor  are  brought 
together.  Such  a  cross  should  show  whether  the  germ- 
cells  of  the  hybrid  are,  as  postulated,  of  two  sorts,  and 
whether  equal  numbers  of  each  sort  are  produced.  Mendel 
made  such  tests,  and  obtained  equal  numbers  of  two  kinds 
of  offspring. 

Mendel  obtained  results  like  these  with  tall  versus 
short  peas  for  other  pairs  of  characters,  such  as  f asciated 
versus  normal  stems,  hard  versus  soft  pod,  yellow  versus 


24 


PHYSICAL  BASIS  OF  HEREDITY 


green  pods,  gray  versus  white -skinned  peas,  yellow  versus 
green  cotyledons  (seen  through  the  skin  of  the  seed), 
and  round  versus  wrinkled  seeds  (determined  by  the 
nature  of  the  cotyledons  within  the  seed  coat). 

The  3 : 1,  Fo,  ratio  characteristic  for  a  single  pair  of 
characters  is  the  expectation  based  on  the  chance  meeting 
of  either  one  of  two  kinds  of  eggs  with  either  one  of  two 
kinds  of  pollen  grains.  In  actual  numbers  this  ratio  is, 
of  course,  not  always  exactly  realized,  but  only  approxi- 
mately. For  the  seven  pairs  of  characters  that  Men- 
del examined,  the  F2  ratios  were  as  follows : 


Dominants 

Receesivee 

No's,  per  4 

Form  of  seed 

7,324 

8,023 
929 

1,181 
580 
858 

1,064 

5,474 

6,022 

705 

882 

428- 

651 

787 

1,850 
2,001 
224 
299 
152 
207 
277 

2.99 

3.00 

3.04 

2.99 

2.95  : 

3.03 

2.92  : 

1.01 

Color  of  cotyledons 

Color  of  seed  coats 

:  1.00 
0.96 

Form  of  pod 

1.01 

Color  of  p>od 

1.05 

Position  of  flowers 

0.97 

Lenjrth  of  stem 

1  ns 

Totals 

19,959 

14,949 

5,010 

2.996  :  1 .004 

The  following  collective  data  for  the  inheritance  of 
color  of  the  cotyledons  of  garden  peas  show  that  the 
approximation  to  a  3  to  1  for  the  recessive  character  is 
very  close ; 


Yellow 

Green 

Total 

No's,  per  4 

Probable  errors 

Mendel 

Correns 

6,022 

1,394 

3,580 

1,310 

11,903 

1,438 

109,060 

1,089 

1,647 

1,012 

3,000 

3,082 

222 

2,405 

50 

2,001 

453 

1,190 

445 

3,903 

514 

36,186 

354 

543 

344 

959 

1,008 

1,856 

70 

850 

8,023 
1,847 
4,770 
1,755 
15,806 
1,952 
145,246 
1,443 
2,190 
1,356 
3,059 
4,090 
7,518 
295 
3,250 

3.002  :  0.998 
3.019  :  0.981 
3.002  :  0.998 
2.986  :  1.014 

3.012  :  0.988 
2.947  :  1.053 
3.004  :  0.996 
3.019  :  0.981 
3.008  :  0.992 
2.985  :  1.015 
3.0;U  :  0.969 
3.014  : 0.986 

3.013  :  0.987 
3.051  :  0.949 
2.954  :  1.046 

±0.0130 
±0.0272 

Tschermak 

Hurst 

±0.0169 
±0.0279 

Bateson 

±0.0093 

Lock 

Darbishire 

Darbyshire 

White 

±0.0264 
±0.0030 
±0.0308 
±0.0250 

Correns 

±0.0319 

Tschermak 

Lock 

±0.0186 
±0.0183 

Darbishire 

Correns 

Lock 

±0.0135 
±0.2151 
±0.0205 

Totals 

218,425 

50,676 

203,500 

3.004  :  0.996 

±0.0026 

M  r  State  CoUeJF 


PARENTS 


Fig.  2. — Cross  between  white  and  red  flowered  four-o'clor-ks  {Mirahilis  jalapa).  In 
the  lower  part  of  the  diagram  the  large  circles  represent  somatic  conditions,  the  included 
small  circles  the  genes  that  are  involved. 


MENDEL'S  FIRST  LAW  25 

That  MendePs  principles  apply  to  animals  was  first 
made  out  by  Bateson  and  by  Cuenot  in  1902.  Since  then 
many  characters  both  in  domesticated  and  in  wild  animals 
and  plants  have  been  studied,  and  there  can  be  no  question 
of  the  wide  application  of  MendePs  discovery. 

During  the  years  immediately  following  the  re-dis- 
coveiy  of  MendePs  principles  (1900)  much  attention  was 
paid  to  the  phenomena  of  dominance  and  recessiveness. 
This  was  due,  no  doubt,  to  the  striking  fact  that  the  hybrid 
sometimes  resembles  only  one  parent  in  some  particular 
trait,  whereas  the  older  observations,  where  many  charac- 
ters were  generally  involved  in  the  cross,  seemed  to  have 
shown  that  hybrids  are  intermediate  in  regard  to  their 
parents.  We  now  know,  however,  that  although  there  are 
cases  in  which  the  dominance  is  as  complete  as  in  those 
described  by  Mendel,  yet  in  a  very  large  number  of  forms 
the  hybrid  is  intermediate  between  the  parents,  even 
when  only  a  single  pair  of  characters  is  involved.  A  few 
examples  will  serve  to  illustrate  these  relations. 

The  common  garden  four  o  'clock,  Mirahilis  jalapa,  has 
a  white-flowered  and  a  red-flowered  variety  (Fig.  2). 
When  crossed,  the  hybrid  has  a  pink  flower,  which  may  be 
said  to  be  intermediate  in  color  between  white  and  red. 
Here  neither  color  can  strictly  be  said  to  dominate.  When 
the  hybrid  (F^)  is  self-fertilized  the  offspring  {F2)  are 
in  the  proportion  of  one  white,  to  two  pink,  to  one  red- 
flowered  plant.  The  Fn  reds  and  the  F2  whites  breed  true ; 
the  pinks  when  self-fertilized  give  white,  pink  and  red  in 
the  proportion  of  1 :  2 : 1.  In  a  case  of  this  kind  the  color 
of  the  F2  plants  reveals  the  nature  of  the  three  classes 
present,  so  that  it  is  not  necessary  to  test  them  out,  as  was 
the  case  in  the  F2  generations  of  MendePs  peas,  where 
the  Fo  tails  were  found  in  this  Avay  to  be  of  two  sorts. 
The  F2  results  w^ith  the  four  o'clock  also  show  that 
the  segregation  of  the  genes  is  clean,  for  the  F2  whites 
never    produce    in    subsequent    generations     anything 


26  PHYSICAL  BASIS  OF  HEREDITY 

but  white  descendants,  and  the  F2  reds  never  anything 
but  red   descendants. 

In  this  case  the  color  of  the  F^  flowers  is  obviously 
somewhere  between  red  and  white.  In  so  far  as  the  F^ 
flower  is  colored,  it  may  be  said  that  red  is  dominant ;  in 
which  case  the  red  and  the  pink  Fn  classes  (1  +  2  =  3) 
are  to  be  counted  together  as  contrasted  wath  the  white, 
giving  a  3 : 1  ratio.  On  tlie  other  hand,  if  one  chose  to 
emphasize  the  fact  that  the  F^  pink  flower  is  not  red,  but 
affected  by  the  white-producing  element  in  its  make-up, 
then  not  red,  but  white,  might  be  said  to  be  the  dominating 
character;  in  which  case  the  white  and  the  pink  F2  classes 
(1  +  2  =  3)  would  be  counted  together  as  contrasted  with 
the  red  giving  an  inverse  3 : 1  ratio.  It  appears  then 
largely  a  matter  of  choice  as  to  what  is  to  be  called 
dominance  (see  below).  The  essential  fact  of  segrega- 
tion is  not  affected  by  the  decision,  and  it  is  this  that  is 
fundamentally  important. 

Another  example  of  failure  of  complete  dominance  is 
shown  in  the  race  of  Andalusian  fowls.  In  this  race  there 
are  blue,  splashed-white,  and  black  birds ;  the  blue  birds 
going  under  the  name  of  Andalusians.  When  splashed- 
white  is  mated  to  black,  all  the  offspring  {F^)  are  blue 
(Fig.  3) ;  when  these  blues  are  bred  together  they  give 
1  splashed-white  :  2  blues  :  1  black.  Evidently  the  blue 
birds  are  the  heterozygous  type.  Their  feathers  show 
under  the  microscope  less  black  pigment,  somewhat  dif- 
ferently distributed  from  that  in  the  black  birds.  The 
intermediate  blue  color  is  due  in  this  case  to  the  less  dense 
distribution  of  the  pigment  in  the  heterozygote.  Lippin- 
cott,  who  has  recently  examined  this  cross  in  greater  detail 
than  heretofore,  states  that  the  colored  areas  or  splashes 
in  the  white  males  are  either  blue  or  blackish  according 
to  the  part  of  the  body  on  which  they  occur,  and  that  this 
corresponds  with  the  distribution  of  the  color  on  the  Anda- 
lusian, for  while  the  latter  is  said  to  be  blue,  this  applies 


i'  ■  ,  ^ 


P, 


Fig.  3. — Cross  between  splashed-white  and  black,  giving  in  Fi  Andalusian,  and  in  Fj  one 
splashed-white,  two  Andalusian,  and  one  black. 


MENDEL  ^S  FIRST  LAW  27 

strictly  only  to  the  hen  and  to  the  lower  parts  of  the  body 
in  the  cock  whose  upper  surface  is  very  dark  blue  or 
even  black. 

In  this  case  neither  black  nor  white  can  be  said  to  be 
dominant.  The  blue  brought  in  as  splashes  by  the 
splashed-white  might  indeed  be  regarded  as  dominant  over 
the  black  of  the  other  (black)  parent,  but  if  so,  then  the 
uniform  distribution  of  the  blue  must  be  determined  by 
dominance  of  the  allelomorphic  gene  brought  in  by  the 
black  parent.  Each  parent  then  would  contribute  at  the 
same  time  a  dominant  and  a  recessive  effect,  each  the 
product  of  one  member  of  the  same  pair  of  allelomorphs. 

There  are  other  cases  in  which  the  hybrid  is  inter- 
mediate in  color,  and,  in  addition,  its  range  of  variation 
is  so  large  that  the  extremes  overlap  one  or  even  both 
of  the  two  parental  types.  For  example :  In  the  vinegar 
fly,  Drosophila  melanogaster,  there  is  a  race  with  ebony 
wings  and  another  race  with  sooty  wings.  Wlien  such 
flies  are  crossed  to  each  other,  the  wings  of  the  F^  fly  are 
intermediate  in  color,  ranging  from  wings  like  those  of 
sooty  to  wings  as  black  as  ebony.  When  the  F^  flies  are  in- 
bred they  give  rise  to  a  series  that  at  one  extreme  has  gray 
wings  and  at  the  other  black  wings.  Separation  into  three 
classes  is  diflicult  or  impossible.  Here  it  may  appear  that 
the  two  original  characters  have  completely  blended  in 
F^  and  in  F^,  but  that  there  are  in  reality  three  classes 
of  flies  in  Fo  can  be  demonstrated  by  suitable  tests.  If, 
for  instance,  we  pick  out  a  sufficient  number  of  i^\  males 
to  give  a  fair  sample  of  the  population,  and  mate  each 
male  first  to  an  ebony  female  of  pure  stock,  and  then  to 
a  female  of  sooty  stock,  we  shall  find  that  one-quarter  of 
the  males  mated  to  ebqny  give  only  ebony,  one-quarter 
mated  to  sooty  give  only  sooty,  while  the  remaining  two- 
quarters  give,  both  in  the  back-cross  to  sooty,  and  in  that 
to  ebony,  a  wider  ranging  group,  which  is  darker  on  the 
whole  when  mated  to  ebony,  and  lighter  when  mated  to 


28 


PPIYSICAL  BASIS  OF  HEREDITY 


sooty.  These  and  other  tests  show  that  in  the  F^  hybrid 
segregation  of  tlie  same  kind  as  in  the  preceding  cases  has 
taken  place,  but  the  results  are  obscured  by  the  wide 
variability  of  the  hybrid  flies.  In  other  words,  evidence 
can  be  obtained  that  the  segregation  of  the  genes  has  been 
clean  cut,  even  although  this  is  obscured  by  the  character 
of  the  heterozygous  flies. 


Fia.  4. — Male  and  female  vinegar  fly  (Drosophila  melanoyaster). 


In  the  iDreceding  illustrations  the  character  difference 
between  the  two  races  is  supposed  to  show  itself  in  the 
same  environment.  It  has  been  found  in  a  few  other 
cases  that  the  dominance  of  one  character  over  the  other 
may  depend  on  the  environment.  For  example,  in  the 
normal  vinegar  fly  the  black  bands  of  the  abdomen  show 
great  regularity  (Fig.  4),  but  in  a  mutant  race  called 
"abnormal  abdomen'^  (Fig.  5)  the  bands  may  be  irregu- 
larly broken  up,  or  even  absent.  In  cultures  with  abund- 
ance of  fresh  food  and  moisture,  all  the  individuals  have 
very  irregular  bands,  but  as  the  culture  gets  old,  and  the 


MENDEL'S  FIRST  LAW 


29 


food  and  moisture  become  less  and  less,  the  bands  become 
more  and  more  regular  until  at  last  the  flies  are  indistin- 
guishable from  normal  flies.  If  a  cross  is  made  between 
a  female  with  abnormal  bands  and  a  wild  male,  the  off- 
spring that  first  hatch  under  favorable  conditions  are  all 
very  abnormal.  Here  abnormal  completely  dominates 
normal  bands.  But  as  the  culture  dries  up,  the  hybrid 
offspring  become  more  and  more  normal,  until  finally  they 
are  all  normal.  At  this  time  it  might  be  said  that  normal 
dominates  abnormal.  Both  statements  are  correct,  if  we 
add  that  in  one  environment  abnormal  banding  dominates, 


Fig.  5. — Normal  and  abnormal  abdomen  of  D.  melanogaster. 

in  another  environment  normal  banding  dominates.  The 
genetic  behavior  of  the  pairs  of  genes  is  the  same  here 
as  in  all  other  cases  of  Mendelian  behavior,  but  this  is 
revealed  onlv  when  the  environment  is  one  in  which  the 
abnormal  gene  produces  one  effect,  the  normal  a  different 
one.  That  the  gene  is  not  itself  affected  by  the  environ- 
ment can  be  shown  very  simply.  If  a  female  from  the 
abnormal  stock  be  picked  out,  at  a  time  when  the  stock 
has  onlv  normal  bands,  and  crossed  to  a  wild  male,  the 
offspring  will  all  be  as  ' '  abnormal ' '  as  when  the  mother 
herself  is  abnormal,  provided  the  food  and  moisture 
conditions  are  of  the  right  kind.  The  late  hatched  normal 
flies  of  abnormal  stock  may  be  bred  from  for  several 


30  PHYSICAL  BASIS  OF  HEEEDITY 

generations,  but  as  soon  as  a  "generation  hatches  under 
favorable  conditions  they  are  as  abnormal  as  though  all 
their  ancestors  had  been  of  this  sort.  Thus  it  is  evident 
that  no  fundamental  importance  is  to  be  attached  to  domi- 
nance of  characters.  On  the  other  hand,  it  is  equally 
obvious  that  it  would  be  entirely  unwarranted  to  suppose 
that  incompleteness  of  dominance  is  due  to  failure  of 
segregation  of  the  genes  that  stand  for  the  characters. 

While  the  problem  of  segregation  can  be  studied  to 
greatest  advantage  where  the  characters  of  a  pair  are 
sharply  separated,  yet  even  where  the  pair  does  not 
possess  this  advantage,  the  cleanness  of  the  segrega- 
tion process  can  be  just  as  definitely,  though  more 
laboriously,  demonstrated. 

In  cases  where  there  is  an  overlap  between  the  hetero- 
zygous type  and  one  of  the  parental  types  it  may,  simply 
as  a  matter  of  convenience,  be  advantageous  to  call  that 
character  that  gives  the  more  continuous  Fo  group  the 
dominant,  thus  leaving  the  smaller  more  sharply  defined 
group  as  the  recessive.  For  example,  the  F2  group  from 
black  by  wild-type  Drosophila  may  be  represented  by 
such  a  scheme  (Fig.  6)  as  the  following: 


Fig.  6. — Relation  of  black  body  color  to  wild  type  as  shown  by  the  claBses  of  Fi  flics. 
The  heavy  outline  includes  the  mutant  class,  the  lighter  line  the  wild  type,  and  the  dotted 
line  the  heterozygous  class. 

Here  the  heterozygous  flies  are  typically  intermediates, 
but  their  variability  overlaps  that  of  the  wild  type  to 
such  an  extent  that  separation  of  the  intermediate  from 
the  wild  type  is  practically  impossible.  On  the  other  hand, 
there  is  no  difficulty  in  making  a  complete  separation 
between  the  heterozygous  class  and  the  homozygous  black. 


MENDEL'S  FIRST  LAW 


31 


Black  is   accordingly  treated  as  a  recessive  in  nearly 
all  experiments. 


a 


at 

Fig.  7. — Normal    eye,  o,  a',  heterozj'gous  eye  h,  V ,  and  bar  eye  c,  c',  of  the  vinegar  fly. 

A  mutant  eye  shape  of  Drosophila,  called  ^^bar''  (Fig. 
7,  a) ,  has  an  intermediate  hybrid  type  (Fig.  7,h),  The  F2 
group  may  be  represented  (Fig.  8)  in  the  following  scheme: 


v._ 


Fig.  S. — Relation  of  bar  eye  to  normal  eye,  as  shown  by  the  Ft  classes. 

In  this  case  the  hybrid,  intermediate  type,  overlaps  the 
bar  type,  so  that  in  Fc  these  two  latter  types  give  a  nearly 
continuous  class.  At  the  other  end  of  the  Fn  series,  the 
round  eyed  normal  (or  wild)  type  can  be  distinguished 
without  difficulty  from  either  of  the  other  classes.  Bar  is 
therefore  normally  treated  as  a  dominant. 


32 


PHYSICAL  BASIS  OF  HEREDITY 


The  case  of  Mirahilis,  or  of  the  Andalusian  fowl,  might 
be  represented  (Fig.  9)  in  the  following  scheme: 


\ 


\ 


FiQ.  9. — Relation  of  Andalusian  to  splashed  white  and  to  black  as  shown   by  classes 

of  f  2  birds. 

Here  all  three  types  are  fully  separable,  in  which  case 
either  homozygote  might  be  considered  the  dominant. 

Finally,  to  return  to  the  case  of  the  tall  and  short 
peas,  the  following  scheme  (Fig.  10)  represents  the  Fg 


Fig,   10. — Relation  of  tall  to  short  peas  as  shown  by  F2  classes. 

group:  Here  the  tall  and  the  heterozygous  group  are 
alike,  and  inseparable  by  ordinary  inspection,  even  at 
the  extreme  end  of  their  variation  curves,  and  short  is 
'  ^  completely  *  *  recessive. 

In  cases  in  which  the  environment  enters  more 
obviously  into  the  result  (as  in ' '  abnormal  abdomen, ' '  Fig. 
5),  the  following  scheme  (Fig.  11)  represents  the  relation : 


Dry 


IVef 


Fia.   11. — Relation  of  normal  to  abnormal  abdomen  as  shown  by  classes  of  F2  flies.    "Dry" 
signifies  conditions  that  make  for  normal;    wet  for  abnormal. 

In  this  case  both  the  heterozygous   and  the   parental 
^^abnonnaP^  type  may  show  *' normal' '  abdomen  like  the 


MENDEL'S  FIEST  LAW  33 

wild  type.  The  abnormal  type  is  treated  as  the  dominant 
although  only  when  the  conditions  are  favorable  to  its 
appearance  is  the  hereditary  phenomenon  seen.  In 
another  case  (duplicate  legs)  only  the  homozygous  form 
may  show  the  duplications  (in  a  special  environment). 
The  following  scheme  (Fig.  12)  represents  this  relation, 
reduplication  of  legs  being  treated  as  a  recessive: 


Fio.   12. — Relation   of  normal  to  duplicate  legs. 

There  are  still  other  relations  that  affect  the  dominance 
of  characters.  For  example,  there  may  be  internal  fac- 
tors, which  when  present,  determine  that  a  character  shall 
be  dominant  over  its  allelomorph,  or  recessive  to  it.  In 
this  connection  might  be  mentioned  what  has  been  called 
'^ reversal  of  dominance.''  An  example  from  Davenport 
will  illustrate  what  is  meant.  In  a  certain  strain  of  fowls 
there  is  a  tendency  for  the  toes  to  be  united  by  a  web  at 
the  base.  Crossed  to  birds  with  normal  feet,  no  birds 
with  united  toes  (syndactyls)  appeared  in  F^.  The  F^ 
birds  inbred  gave  in  Fo  only  about  10  per  cent,  of  s}Tidactyl 
birds.  It  would  appear  that  the  latter  character  is  reces- 
sive, and  that  the  recessive  type  overlaps  largely  the 
dominant  heterozygous  type. 

Davenport  interpreted,  however,  the  syndactyl  as  the 
dominant  type,  because  ^Hwo  syndactyls  may  give  nor- 
mals, but  no  true  normals  give  syndactyls."  In  other 
words,  he  defines  the  dominant  type  as  the  one  that  can 
carry  the  other  type,  because  he  says  dominance  is  due  to 
presence  of  factors,  recessiveness  to  absence.  *^Now 
dominance  may  fail  to  develop  but  recessiveness  never 
can  do  so.*'  For  this  reason  two  syndactyls  may  give 
3 


34  PHYSICAL  BASIS  OF  HEREDITY 

normals, because  a  dominant  cliaracter  may  fail  to  develop, 
even  though  its  factors  be  present.  Since  normal  feet 
never  give  syndactyls,  the  normal  type  must  be  recessive. 
But  Davenport's  definition  of  a  recessive  type  as  one 
that  never  shows  in  the  heterozygous  condition  is  in  my 
opinion  based  on  an  arbitrary  distinction  of  what  is  the 
cause  of  dominance  and  recessiveness.  The  evidence  may, 
I  think,  be  better  interpreted  as  indicated  in  the  same 
diagram  as  that  for  abnormal  abdomen  (Fig.  11)  in  that 
part  marked  "dry,"  in  which  the  syndactyl  condition 
would  be  represented  as  recessive  (heavy  line).  In  the 
hybrid  the  character  is  usually  seen  only  in  a  few  individ- 
uals, i.e.,  it  is  intermediate,  overlapping  both  parent  types. 
While  this  case  shows  that  it  is  often  only  a  convention 
as  to  which  type  is  called  the  dominant  and  which  the 
recessive,  I  can  see  no  special  reason  why  in  these  cases 
of  syndactylism  the  usual  convention  may  not  be  followed 
.which  recognizes  the  small  F2  class  as  the  recessive. 

Mendelism  rests  on  the  theory  of  a  clean  separation 
of  the  members  of  each  pair  of  factors  (genes).  In 
every  heterozygote  the  factor  for  the  dominant  and  that 
for  the  recessive  are  supposed  to  come  into  relation  to 
each  other  and  then  to  separate  at  the  ripening  of  the 
germ-cells.  If  we  think  of  the  two  genes  coming  together 
and  afterwards  separating,  it  w^ould  seem  that  a  favor- 
able situation  might  exist  for  the  two  to  become  mixed, 
and  one  "contaminate"  the  other.  If  any  extensive 
process  of  this  kind  occurred  the  Mendelian  phenomena 
would  be  so  irregular  and  erratic  that  they  would  have 
little  interest.  But  even  those  who  are  inclined  to  appeal 
to  contamination  as  an  exceptional  phenomenon,  grant 
that  clean  separation  of  the  genes  is  the  rule.  The  best 
critical  evidence  against  contamination  is  in  cases  in  which 
for  many  successive  generations  breeding  has  taken  place 
from  heterozygous  forms  only  (which  creates  a  favorable 
situation  for  contamination  to  take  place  w^ere  it  possible). 
No  influence  of  contamination  has  been  found  in  such  cases. 


MENDEL'S  FIRST  LAW 


35 


Marshall  and  Muller  kept  flies  heterozygous  for  three  re- 
cessive mutant  factor  for  about  seventy-five  generations, 
and  at  the  end  of  that  time  found  that  these  factors  had 
not  been  weakened  in  any  way  as  a  result  of  juxtaposition 


Fig.   13. — Notch  wings  in  the  vinegar  fly,   extreme  condition,   a;    average    condition,   6; 

nearly  normal  condition,  c. 

with  their  normal  dominant  allelomorphs.  I  have  kept 
a  stock  of  notch-winged  flies  under  selection  for  twenty- 
five  generations.  Notch  (Fig.  13)  is  a  character  varying 
in  the  direction  of  normal  wings  (Fig.  13,  c) ;  in  eveiy 
generation  of  notch,  many  notch  flies  have  normal  wings. 
The  character  is  dominant,  and  exists  only  in  heterozy- 


36  PHYSICAL  BASIS  OF  HEREDITY 

gous  condition,  since  a  fly  homozygous  for  notch  dies.  The 
race  is  therefore  necessarily  maintained  in  a  hetero- 
zygous state.  In  each  generation  females  that  were 
genetically  notch,  but  had  normal  wings,  were  selected 
and  bred  to  normal  males.  Tlie  selection  was  away  from 
notch  (i.e.,  toward  normal).  After  a  tune  more  than  half 
of  the  notch  flies  had  nonnal  wings.  The  effect  produced 
proved  to  be  due  not  to  a  change  in  the  notch  gene  through 
contamination,  but  to  modifying  genes;  for  at  the  end  of 
the  selection  tlie  original  notch  could  be  recovered  at  any 
time  by  removing  the  influence  of  the  modifying  factor. 
It  has  been  sometimes  stated,  usually  by  the  opponents 
of  Mendel's  theory,  or  by  advocates  of  doctrines  of  evolu- 
tion that  appeared  to  be  compromised  by  the  Mendelian 
conception  of  *^unit  factors,'*  that  Mendelism  deals  only 
with  such  superficial  characters  as  the  color  of  flowers 
or  the  hair  color  of  mammals.  This  statement  contains 
an  element  of  truth  in  so  far  as  it  covers  most  of  the 
kinds  of  characters  that  students  of  heredity  find  most 
convenient  to  study;  but  it  contains  an  entirely  false 
inference  as  to  the  limitations  of  Mendelism.  The  issue 
involved  is  this :  changes  in  superficial  characters  are  not 
so  likely  to  affect  the  ability  of  the  organism  to  survive 
as  are  changes  in  essential  organs ;  hence  they  are  the  best 
kind  of  hereditary  characters  for  study.  But  there  is  no 
evidence  that  such  superficial  characters  are  inherited  in 
a  different  way  from  "fundamental"  characters,  and 
there  is  evidence  to  the  contrary.  A  common  class  of 
characters  showing  perfect  Mendelian  behavior  are 
so-called  lethals  that  destroy  the  individual  when  in  homo- 
zygous  condition.  There  can  be  no  question  as  to  the 
fundamental  importance  of  such  factors.  Between  these 
extreme  cases  and  the  superficial  shades  of  eye  color, 
for  example,  all  possible  gradations  of  structure,  physio- 
logical and  pathological,  are  known.  The  only  possible 
question  that  might  be  seriously  raised  is  whether  these 
characters  are  all  losses  or  deficiencies,  while  progres- 


MENDEL'S  FIRST  LAW  37 

sive  advances  may  belong  to  a  different  category.  This 
may  be  a  serious  question  for  the  evolutionist,  but  has 
nothing  to  do  with  the  problem  that  concerns  us  here. 

In  recent  years  an  entirely  unexpected  and  important 
discovery  in  regard  to  segregating  pairs  of  genes  (allelo- 
morphs) has  been  made.  In  an  ever-increasing  number  of 
cases  it  has  been  found  that  there  may  be  more  than 
two  distinct  characters  that  act  as  allelomorphs  to  each 
other.  For  example,  in  mice,  yellow,  sable,  black,  wliite- 
bellied  gray,  and  gray-bellied  gray  (wild  type)  are  allelo- 
morphs, i.e.,  any  two  may  be  present  (as  a  pair)  in  an 
individual,  but  never  more  than  two.  In  DrosopJiila  the 
eye  colors  white,  eosin,  cherry,  blood,  tinged,  buff,  milk, 
ivory,  coral  and  the  normal  allelomorph  form  a  series  of 
multiple  allelomorphs.  In  the  grouse  locust,  Paratettix, 
there  are  nine  types  that  may  be  allelomorphic,  all  of 
which  exist  in  the  wild  state  (Nabours).  In  Drosophila, 
again,  there  are  as  many  as  twelve  other  series  of  allelo- 
morphs known  at  present ;  in  rats  there  is  a  small  allelo- 
morphic series,  also  two  in  guinea  pigs  and  two  in  rabbits. 
In  plants  there  are  a  few  cases  known,  especially  in  corn. 
In  all  these  series  it  is  the  same  organ  that  is  mainly 
affected  by  the  different  allelomorphs,  which  seems  ''natu- 
ral, ' '  but  was  not  necessarily  to  have  been  expected.  The 
chief  interest  of  these  series  is  that  they  appear  to  demon- 
strate that  the  normal  (wild  type)  allelomorph,  and  its 
mutant  mates  need  not  be  due  to  presence  and  absence, 
but  rather  represent  modifications  of  the  same  unit  in  the 
hereditary  material ;  for,  taken  literally,  only  one  absence 
is  thinkable,  and  yet  in  Drosophila  there  are  eight  such 
"absences''  in  one  series. 

As  has  been  stated,  Mendel  did  not  make  it  clear  that 
there  exists  in  the  normal  animal  or  plant  the  same  dual- 
ity that  comes  to  light  when  a  hybrid  is  produced ;  never- 
theless this  condition  is  implied,  at  least,  in  his  paper, 
and  has  been  taken  for  granted  in  practically  all  of  the 
modern  w^ork  on  heredity.    The  demonstration  that  such 


38  PHYSICAL  BASIS  OF  HEEEDITY 

is  the  case  is,  however,  not  a  simple  matter.  It  could  not 
have  been  made  by  Mendel  or  in  the  earlier  days  after  the 
rediscovery  of  Mendelism  (1900).  An  attempt  to  furnish 
this  demonstration  is  given  in  Chapter  XX.  Assuming 
the  demonstration  to  be  satisfactory,  we  reach  the  highly 
important  conclusion  that  segregation  is  not  something  m 
peculiar  to  hybrids,  but  something  most  readily  demon- 
strated by  means  of  hybrids,  and  that  in  all  probability  the 
germ-plasm  is  at  first  made  up  of  pairs  of  elements,  but 
at  the  ripening  of  the  germ-cells  these  elements  (genes) 
separate,  one  member  of  each  pair  going  to  one  daughter 
cell,  the  other  member  to  the  other  cell.  The  mechan- 
ism by  means  of  which  such  a  process  might  take  place 
had  been  known  for  several  years  before  its  relation 
to  Mendel's  principles  of  segregation  was  realized.  This 
mechanism  is  to  be  found  in  the  conjugation  and  reduc- 
tion processes  that  take  place  in  the  maturation  of  egg- 
and  sperm-cell.  An  account  of  this  process  is  given  in 
the  next  chapter. 


CHAPTER    III 
THE  MECHANISM  OF  SEGREGATION 

One  of  tlie  most  secure  generalizations  of  modern  work 
on  the  cell  is  that  every  cell  of  the  individual  contains  a 
constant  niimher  of  self-perpetuating  bodies  (called  chro- 
mosomes), half  of  which  are  traceable  to  the  father  and 
half  to  the  mother  of  the  individual.  No  matter  how 
specialized  cells  may  be,  they  contain  the  same  number 
of  chromosomes.  Equally  important  is  the  fact  that  after 
the  eggs  of  the  female  and  the  sperm-cells  of  the  male 
have  passed  through  the  ripening  or  maturation  divisions 
the  number  of  chromosomes  is  reduced  to  half.^  Lastly, 
there  is  convincing  evidence  that  the  reduced  number  of 
chromosomes  is  brought  about  as  the  result  of  a  separa- 
tion of  such  a  kind  that  each  mature  germ-cell  gets  only  a 
paternal  or  a  maternal  member  of  each  chromosome  pair. 

The  reduction  takes  place  in  the  female  at  the  time 
when  the  polar  bodies  are  given  off  from  the  Qgg ;  and  in 
the  male  just  prior  to  the  formation  of  the  spermatozoa. 
A  characteristic  process  is  seen  in  the  oogenesis  and  sper- 
matogenesis of  the  nematode  worm  Ancyr acanthus  cysti- 
dicola  (a  parasite  in  the  swim-bladder  of  fresh-water 
fishes)  described  by  Mulsow.  The  young  eggs  contain 
twelve  chromosomes  (Fig.  14,  a).  As  the  result  of  the 
later  union  of  these  twelve  in  pairs,  six  short  threads 
appear  in  the  nucleus  of  the  Qg^  just  before  it  extrudes  its 
polar  bodies.  The  threads  contract  to  six  short  rods 
(split  in  two  planes  at  right  angles  to  each  other),  the 
tetrads  (Fig.  14,  c).  With  the  dissolution  of  the  nuclear 
wall  these  tetrads  are  set  free  in  the  protoplasm,  and  a 
spindle  develops  about  them  (Fig.  15,  a).  They  pass  to 
the  equator  of  the  spindle,  and  there  dividing  lengthwise, 

*  Exceptions  occur  in  certain  cases  of  parthenogenesis. 

39 


40 


PHYSICAL  BASIS  OF  HEREDITY 


half  of  each  goes  to  one  pole,  and  half  to  the  other  pole 
of  the  spindle  (Fig.  15,  h).  One  end  of  the  spindle  pro- 
trudes from  the  egg,  and  around  it  the  protoplasm  con- 


a 


■fr 


Fig.   14. — 06cyte  of  Ancyr acanthus,  a;  growth  period,  b;  nucleus  with  tetrads,  c.      (After 

Mulsow.) 

stricts  off  (Fig.  15,  c)  to  form  the  first  polar  body.  About 
the  six  ovoidal  chromosomes  left  in  the  egg  a  new  spindle 
develops;  and  these  chromosomes  become  drawn  into 
its  equator,  where  they  divide  again,  half  of  each  going 


a 


c 


e 


Fig.  15. — Egg  of  Ancyr  acanthus  with  six  tetrads,  a;  egg  with  first  polar  spindle,  h; 
egg  after  extrusion  of  first  polar  body,  c;  egg  with  second  polar  spindle,  d;  egg  after  the 
extrusion  of  both  polar  bodies,  e. 

to  one  pole  and  half  to  the  other  (Fig.  15,  d).  A  sec- 
ond protrusion  takes  place  from  the  surface  of  the  egg 
which  pinches  off  to  form  the  second  polar  body  (Fig. 
15,  e).  Thus,  after  two  mitotic  divisions,  the  egg  has  lost 
three-quarters  of  its  chromatin,  but  retains  half  the  full 


MECHANISM  IN  SEGREaATION 


41 


nmnber  of  chromosomes,  and  as  a  result,  tlie  original 
twelve  chromosomes  have  been  reduced  to  six. 

Around  the  six  chromosomes  left  in  the  egg,  a  nuclear 
wall  forms,  and  the  chromosomes  become  spun  out  into 
delicate  fibres.  Meanwhile  a  spermatozoon  has  entered 
the  eggj  and  out  of  its  head  another  nucleus  develops.  The 
two  nuclei,  the  egg  nucleus  and  the  sperm  nucleus,  move 
toward  the  center  of  the  egg  (Fig.  16,  a),  where  they 
come  into  contact  with  each  other.  After  a  time,  the 
chromatin  threads  begin  to  condense  again  into  rods. 


Fig  16. — Eggs  of  Ancyracanthus  within  membrane.  Egg  with  two  pronuclei,  a; 
egg  pronucleus  with  six  chromosomes  and  sperm  nucleus  with  six  chromosomes,  h;  egg  pro- 
nucleus with  six  chromosomes  and  sperm  nucleus  with  five  chromosomes,  c;  union  of  male 
and  female  pronuclei,  d.      (After  Mulsow.) 

Six  appear  in  the  egg  nucleus,  and  six  in  the  male  nucleus 
(Fig.  16,  ty.  A  spindle  develops  in  the  protoplasm  of 
the  egg  around  the  twelve  chromosom.es  of  which  six  have 
come  from  the  father  (the  paternal  chromosomes)  and 
six  from  the  mother  (the  maternal  chromosomes)  (Fig. 
16,  d).  Each  chromosome  now  splits  lengthwise  into 
equivalent  halves,  and  a  half  moves  to  each  pole  of  the 
mitotic  spindle.  The  spindle  rotates  in  the  cytoplasm  of 
this  egg  until  its  long  axis  corresponds  with  that  of  the 
egg.  As  the  daughter  chromosomes  move  towards  the 
poles  of  the  mitotic  spindle  the  egg  protoplasm  constricts 

*  Assuming  a  female  producing  sperm  to  have  entered. 


42 


PHYSICAL  BASIS  OF  HEREDITY 


between  them  so  that  two  cells  are  formed,  each  cell  con- 
taining twelve  chromosomes,  six  paternal  and  six  mater- 
nal. Thus,  through  fertilization,  the  whole  number  of 
chromosomes  is  restored  to  the  egg.  This  number  remains 
through  all  subsequent  divisions  of  the  cells  of  the  embryo. 
The  male  of  Ancyracantlius  has  only  eleven  (Fig.  17,  a) 
chromosomes ;  because  the  male  has  only  one  sex-chromo- 


a 


Fig.  17. — Spermatogenesis  of  Ancyracanthus.  Spermatogonia!  cell,  o;  cell  after  growth 
period  with  tetrads,  h;  first  spermatocyte  division,  c;  two  cells  resulting  from  first  division 
with  six  and  with  five  chromosomes,  respectively,  d;  four  cells  resulting  from  the  next 
division,  c;  ditto,  /;  mature  spermatozoa,  one  with  six,  the  other  with  five,  chromoBomes.g; 
ditto,  living  spermatozoa,  h.      (After  Mulsow.) 

some,  while  the  female  has  two  sex-chromosomes.  Both 
sexes  have  ten  other  chromosomes,  sometimes  called  auto- 
somes. Just  before  the  maturation  divisions  take  place, 
there  are  six  rods  in  each  sperm-cell,  five  of  which  (the 
autosomes)  condense  into  tetrads,  the  sixth  (the  sex- 
chromosome)  into  only  a  double  body  (Fig.  17,  &).  A 
spindle  develops  about  these  and  each  of  the  five  auto- 
somes divides.  The  sex-chromosome  does  not  divide,  but 
passes  to  one  pole  of  the  spindle  (Fig.  17,  c).     The  result 


MECHANISM  IN  SEGREGATION  43 

is  that  two  cells  are  produced,  one  with  six,  the  other 
with  five  chromosomes  (Fig.  17,  d). 

Without  a  resting  stage  a  new  spindle  develops  in  each 
cell,  and  a  new  division  takes  place — each  dumb-bell- 
shaped  body  dividing,  as  well  as  the  sex  chromosome  in 
the  cell  that  contains  it.  In  all,  four  cells  result  (Fig.  17, 
e  and  /) — two  with  five  chromosomes  each,  two  with  six 
each.  Each  becomes  a  spermatozoon,  which  in  this  worm 
is  a  round  cell  with  the  chromosomes  at  one  pole  (Fig. 
17,  g).  Half  of  the  spermatozoa  contain  six,  half 
five  chromosomes.  They  can  be  distinguished  even  in 
the  living  sperms  (Fig.  17,  h).  If  a  six-chromosome 
sperm  fertilizes  an  egg  (Fig.  16,  b),  a  female  (with  12 
chromosomes)  is  produced — if  a  five-chromosome  sperm 
fertilizes  an  egg  (Fig.  16,  c),  a  male  (with  11 
chromosomes)  is  produced. 

The  two  chromosome  divisions  (or  separations)  that 
take  place  when  the  polar  bodies  are  extruded  from  the 
egg  are,  for  a  number  of  reasons  that  need  not  be  entered 
into  here,  generally  regarded  as  equivalent  to  the  two 
final  divisions  in  the  ripening  of  the  sperm-cells.  One 
of  the  two  divisions  is  interpreted  as  an  ordinary  cell- 
division  in  which  the  chromosomes  split  lengthwise  into 
equivalent  halves — half  going  to  each  pole.  The  other 
division  is  interpreted  as  a  separation  of  whole  chromo- 
somes that  have  come  together  side  by  side  at  an  earlier 
stage.  The  tetrad  is,  then,  looked  upon  as  a  pair  of 
chromosomes  that  have  conjugated  in  the  sense  that  they 
have  come  to  lie  side  by  side  (with  interchange  of  mate- 
rials at  times  in  a  way  to  be  described  later).  One  split 
is  supposed  to  correspond  to  the  line  between  the  conju- 
gated pairs ;  the  other  split  represents  a  division  in  each 
chromosome  of  the  pair.  As  a  consequence  when  the 
chromosomes  move  apart  (at  the  maturation  division)  one 
of  the  two  divisions  is  said  to  be  a  *^ reducing  division** 
because  whole  chromosomes  are  supposed  to  separate ;  the 
other  division  is  said  to  be  an  ^^  equation  di\dsion,**  each 


44  PHYSICAL  BASIS  OF  HEREDITY 

chromosome  splitting  lengthwise  into  equivalent  halves  as 
in  ordinary  cell-division. 

The  interpretation  of  these  two  divisions  that  occur 
in  the  egg  and  in  the  sperm-cell  has  been  the  subject  of 
much  speculation.  It  is  apparent  that  the  process  reduces 
the  number  of  chromosomes  by  half,  and  that  the  whole 
number  is  regained  by  fertilization.  It  is  sometimes  said 
that  the  "purpose"  of  this  division  is  to  keep  the  number 
of  chromosomes  constant,  for,  if  not  reduced,  they  would 
increase  in  number  with  each  fertilization. 

The  '^reason"  for  the  other,  the  second,  division  is 
acknowledged  to  be  obscure.  For  present  purposes  it  is 
futile  to  speculate  concerning  these  two  divisions,  but  it 
should  be  pointed  out  here  that  the  genetic  evidence  is  in 
full  accord  with  the  interpretation  of  these  two  divisions 
that  is  generally  accepted  to-day  by  cytologists,  i.e.,  that 
one  of  the  divisions  separates  the  conjugating  pair,  and 
that  the  other  represents  a  longitudinal  division  within  a 
paternal  and  within  a  maternal  chromosome  of  each  pair. 

If  w^e  follow  the  history  of  the  germ-cells  further  back 
before  the  maturation  divisions,  we  find  that  between  the 
stage  when  the  half  number  of  chromosomes  reappears 
(tetrads)  and  the  stage  at  which  the  full  number  was 
present,  there  is  a  very  obscure  period  in  the  history  of 
the  germ-cells.  This  period  has  been  studied  chiefly  in 
the  male.  Only  a  few  types  have  been  found  favorable 
for  the  study  of  this  period.  One  of  the  most  favorable 
ones  is  a  marine  annelid,  Tomopteris,  studied  by  the 
Schreiners.  The  early  division  of  the  germ-cells  (the 
spermatogonia)  of  Tomopteris,  when  the  full  number 
of  chromosomes  is  present,  is  show^n  in  Fig.  18,  a-g. 
The  division  is  like  that  of  all  the  other  cells  of  the  body. 
The  chromosomes  appear  as  thick  bent  threads  that  split 
lengthwise  (Fig.  18,a,  b).  The  nuclear  wall  disappears  and 
a  spindle  appears  near  the  group  of  split  chromosomes 
(Fig.  18,  c).  As  the  poles  of  the  spindle  move  apart  the 
chromosomes  become  arranged  at  the  equator  of  the  spin- 


MECHANISM  IN  SEGEEGATION 


45 


die,  each  half  of  each  chromosome  becoming  attached  by 
a  spindle  fibre  to  one  pole  (Fig.  IS^  d).     The  halves  move 


Fig.   18.     Last  spermatogonia!    division    of    Tomopteris,   a-h;    stages  before  and  during 

synapsis,  i-l.      (After  Schreiner.) 

apart  towards  their  respective  poles  (Fig.  18,  e)  and  as 
they  become  separated  into  two  groups  the  cell  protoplasm 


46  PHYSICAL  BASIS  OF  HEREDITY 

constricts  between  them  to  produce  new  cells  (Fig.  18,/). 
When  the  chromosomes  have  reached  the  pole  they  shorten 
(Fig.  18,  g)  and  appear  to  send  out  anastomosing  threads. 
Around  this  group  of  threads  a  new  nuclear  wall  is  formed 
(Fig.  18, /i).  All  trace  of  the  separate  chromosomes  is 
now  lost,  but  between  the  last  stage  just  described  and  the 
stage  now  to  be  described  it  is  supposed  that  important 
changes  in  the  chromosomes  take  place.  This  new  phase 
is  spoken  of  as  the  synapsis  stage.  At  the  beginning  of 
this  stage  (Fig.  18,  i  and  j)  faint  indications  of  the  chromo- 
some appear,  and  soon  they  can  be  seen  again  (Fig.  18,  A:) 
as  long  thin  threads  whose  free  ends  place  themselves  in 
parallel  pairs.  The  pairing  of  the  threads  continues  to 
extend  inwards  from  the  ends  (Fig.  18, 1)  until  they  have 
united  throughout  the  length  of  the  loops  (Fig.  19,(2-). 
There  are  exactly  half  as  many  of  these  loops  as  there 
were  original  chromosomes,  which  is  expected  if  they  have 
united  in  pairs.    The  conjugation  has  been  accomplished. 

During  the  stages  that  follow,  the  double  chromosomes 
shorten  and  become  thicker  (Fig.  19,h,c,d),  and  con- 
dense into  the  form  of  tetrads  (Fig.  19,  e).  They  begin 
to  separate  into  halves,  each  half  is  also  split  lengthwise. 
A  spindle  appears,  and  the  cells  divide  (Fig.  19,  f,g,h). 
In  each  cell  the  chromosomes  show  indications  of  passing 
into  a  resting  stage,  as  happens  after  all  ordinary  cell 
divisions,  but  before  this  change  has  gone  very  far  a  new 
spindle  appears  (Fig.  19,  ^),  and  preparations  for  another 
division  are  rapidly  made.  The  new  division  completes 
the  maturation  of  the  sperm-cells  (Fig.  19,  j,  k,  I).  Each 
of  the  four  cells  resulting  from  the  original  sperm-mother- 
cell  differentiates  into  a  spermatozoon. 

In  one  of  the  salamanders,  Batrocoseps,  the  matura- 
tion stages  of  the  male  are  particularly  well  shown.  The 
essential  stages  in  synapsis  are  shown  in  Fig.  20,  a-d 
as  worked  out  by  Janssens.  These  stages  are  essentially 
the  same  as  those  of  Tom,opteris.  During  the  early  multi- 
plication stages  the  cells  of  the  future  testes  divide  by 


i 


MECHANISM  IN  SEOEEGATION 


47 


the  ordinary  mitotic  process.    The  cells  then  pass  into  the 
synaptic  stage  (Fig.  20,  a-d).     As  the  chromosomes  begin 


a 


g 


Fig.   19. — Thin-thread  stage  of  Tomopferis  spermatocyte,  a-d;    tetrads,  e;    first   sperma- 
tocyte division,   /-/;   second  spermatocyte  division,./-/.      (After  Schreiner). 

to  emerge  as  thin  threads,  it  is  found  in  Batracoseps  that 
their  ends  are  all  pointed  towards  one  pole  (Fig.  20,  d)- 
This  is  the  same  pole  as  that  towards  which  the  two  ends 


48 


PHYSICAL  BASIS  OF  HEREDITY 


of  each  V-shaped  chromosome  pointed  as  the  cell  went 
into  the  resting  stage.  It  appears  then  that  the  chromo- 
somes not  only  retain  tlieir  original  orientation,  but  that 
the  ends  of  homologous  chromosomes  have  already  come 


a 

r:: 


<^ti<i^    /    t 


•v 

Fig.  20. — Synaptic  stages  and  those  immediately  following  in  Batracoseps.    (After  Janssens.) 

together,    or    are    coming    together,    as    the    following 
stages  show  clearly. 

The  union  that  begins  at  the  ends  (Fig.  20,  e)  grad- 
ually extends  along  the  length  of  the  chromosomes,  which 


MECHANISM  IN  SEGREaATION  49 

are  now  in  the  form  of  thin  threads.  At  the  point  where 
the  two  threads  come  together  (Fig.  20,  /)  they  can  often 
be  seen  to  be  shaped  like  a  Y  and,  at  the  point  of  meeting, 
the  uniting  threads  are  often  twisted  about  each  other. 

The  fused  part  of  the  united  threads  steadily  grows 
shorter  and  thicker.  They  become  the  condensed  pachy- 
tene threads,  and  appear  as  represented  in  Fig.  20,  g.  The 
thick  threads  shorten  further,  and  the  line  of  fusion 
between  them  (or  a  new  line  of  cleavage)  appears,  as 
seen  in  Fig.  20,  h.  It  will  be  noticed  also  that  the  ragged 
outline  that  the  chromosomes  had  during  the  preceding 
stages  is  gradually  lost,  so  that  they  now  appear  as  solid 
rods  or  cords,  which  finally  when  they  have  reached  the 
last  stage  in  their  condensation  (Fig.  20,  i)  appear  (in 
Batracoseps)  as  rods  tivisted  about  each  other.  Whether 
this  twisting  represents  the  original  wrapping  around 
each  other  of  the  leptotene  threads  as  they  conjugate,  or 
whether  it  is  a  new  arrangement  resulting  from  the  con- 
densation of  the  chromosomes  that  are  not  free  to  move  at 
all  points,  hence  twist  about  each  other  as  they  condense, 
is  a  question  that  calls  for  further  and  careful  considera- 
tion. For  the  present — since  segregation  alone  is  here 
involved — this  matter  may  be  laid  aside.  In  this  con- 
densed condition  the  chromosomes  pass  into  the  first 
maturation  division. 

As  already  stated,  the  union  of  the  chromosomes  in 
the  eggs  of  the  female  has  been  less  often  studied,  but 
that  the  process  is  essentially  the  same  is  sufficiently  evi- 
dent. In  one  of  the  sharks,  Pristiurus  melanostomus,  the 
following  stages  described  by  Marechal  show  how  similar 
are  the  maturation  stages  in  the  female  to  those  in  the 
male.  When  the  germ-cells  have  reached  the  end  of  the 
multiplication  period  they  pass  into  the  synaptic  condition, 
as  shown  in  Fig.  21,  a  to  d.  Then  threads  appear  in  the 
nucleus ;  and  soon  it  becomes  evident  that  most  of  them 
are  in  the  form  of  loops,  whose  ends  are  uniting  in  pairs 
(Fig.  21,  e,  /).    When  conjugation  is  finished  thick  loops 

4 


50  PHYSICAL  BASIS  OF  HEEEDITY 

are  present  that  shorten  further  into  thick  rods  (Fig. 
21,  g)  that  often  show  a  single  longitudinal  split.  The 
egg  now  begins  to  accumulate  the  enormous  amount  of 
yolk  characteristic  of  selachian  eggs ;  and  during  this  time 
the  chromosomes  become  more  and  more  indistinct.  As 
shown  in  the  figure  (Fig.  21,  h-k)  they  appear  to  send  out 
loops  laterally,  which  loops  may  be  only  the  bendings  of  a 
long  thread.  When  the  yolk  formation  is  finished  the 
chromosomes  condense  into  shorter  threads,  with  lateral 
branches  (Fig.  21, 1).  When  the  egg  is  ripe,  the  nuclear 
wall  is  absorbed,  the  chromosomes  appear  as  short  rods 
(arranged  in  twos),  which  place  themselves  in  the  polar 
spindle.  Two  polar  bodies  are  given  off,  leaving  the 
reduced  number  of  chromosomes  in  the  egg. 

It  is  obvious  from  the  preceding  account  that  the  sperm 
and  the  egg  pass  through  essentially  the  same  stages  dur- 
ing maturation,  the  essential  feature  of  which  is  the  con- 
jugation of  homologous  chromosomes  followed  by  their 
subsequent  segregation.  Each  sperm  and  each  egg  is  left 
with  half  the  original  number  of  chromosomes — one  of 
each  kind. 

Latekal  Versus  End-to-End  Fusion  of  the  Chromosomes 

In  the  preceding  account  of  the  union  of  the  chromo- 
somes only  one  method  of  union  is  described,  viz.,  side-to- 
side  conjugation.  The  tetrad  as  represented  is  due  to  one 
division  plane  between  the  conjugating  pairs,  and  the  other 
due  to  a  longitudinal  split  of  each  conjugating  member. 
But  according  to  some  observers,  more  especially  botan- 
ists, another  method  of  union  also  occurs,  in  which  the  split 
chromosomes  unite  end  to  end.  If  the  division  planes 
in  such  a  tetrad  represent  respectively  the  plane  of  union 
at  the  ends,  and  the  longitudinal  split  through  the  united 
rods,  the  final  result  of  this  separation  would  be  exactly 
the  same  so  far  as  the  four  elements  of  the  tetrad  are 
concerned,  but  the  process  would  have  serious  conse- 


I* 


m. 


'::« 
.-j^ 


/S' 


.:  •  '^^^^ 


,# 

'^^5*^^ 


r 


.■itek 


■■frS^ivJ 


^?S<^fr-N;: 


S^^:^ 


y 


:*,%l^i'^ 


f 


h 


*Ma 


<? 


->   *•■  - 


."»>><■.. 


^ 


^^^V^ 


y 


.'1?, 


^^  ^^i^ 


•  • 


-'^-'•r 


#• 


.■•-•-K  ■*^^ 


5^     • 


-r^ 


:%■ 


y^ 


•  • 


^^^ 


/ 


Fig.  21. — Synaptic  stages,  and  those  immediately  following,  in  the  egg  of  Pristiurus. 

(After  Mar6chal.) 


MECHANISM  IN  SEGREGATION  51 

quence  for  genetics  in  so  far  as  the  chromosomes  represent 
the  bearers  of  genes,  for  while  side-to-side  union  offers 
an  opportunity  for  interchange  between  the  paternal  and 
maternal  members  of  a  pair,  no  such  interchange  could  be 
postulated  if  end-to-end  conjugation  took  place.  So  far 
as  segregation  is  concerned  either  method  supplies  all 
that  is  called  for.^  A  discussion  of  other  matters  will 
be  left  until  later. 

Individuality  of  the  Chromosomes 

During  the  period  of  cell-division  there  can  scarcely 
be  any  question  concerning  the  persistence  of  the  individ- 
ual chromosomes,  because  they  remain  visibly  distinct 
elements  in  the  cell;  but  when  the  nucleus  re-forms  after 
each  division  the  chromosomes  spin  out  threads  laterally, 
and  these  appear  to  fuse,  making  a  continuous  network 
throughout  the  nucleus.  Whether  there  is  actual  fusion 
between  these  threads  or  whether  they  occupy  delimited 
contact  areas,  and  whether  the  branches  represent  the 
essential  part  of  the  chromosome  concerned  in  heredity, 
are  questions  impossible  to  answer  at  present.  The 
genetic  evidence  at  least  consistently  shows  that  no  real 
fusion  of  the  hereditary  material  occurs  even  in  cells  that 
have  passed  through  many  such  resting  periods. 

From  several  other  sources  there  are  strong  indica- 
tions that  the  chromosomes  retain  their  individuality  dur- 
ing the  resting  stage.  In  Ascaris,  where  the  chromosomes 
are  few  and  long,  they  are  often  drawn  out  in  an  irregular 
way  in  the  cleavage  cells  as  they  pass  to  the  poles  of  the 
spindle  of  the  dividing  cells.  Daughter  halves  of  the  same 
chromosomes  show  the  same  identical  irregularity. 
Boveri  has  shown  by  an  examination  of  a  large  number  of 
daughter  cells  (pairs)  that  are  getting  ready  for  the  next 
division,  that  when  the  chromosomes  of  sister  cells  reap- 

®  If  the  pairs  fused  end  to  end  and  the  tetrad  arose  by  two  loncritudinal 
divisions,  the  outcome  would  not  be  in  harmony  with  the  theory  of  segrega- 
tion based  on  separation  of  maternal  and  paternal  chromosomes  at  reduction. 


52 


PHYSICAL  BASIS  OF  HEEEDITY 


pear  they  show  the  identical  irregularities  (Fig.  22,  a 
to  d) .  It  is  probable,  therefore,  that  each  chromosome  has 
retained  the  particular  form  that  it  had  when  it  passed  into 
the  resting  stage ;  or  at  least  that  the  axial  thread  from 
which  the  network  was  spun  out  has  remained  in  place. 

In  a  few  cases  the  chromosomes  appear  more  or  less 
visible  during  the  resting  stages.  This,  however,  is  such 
a  rare  event  that  it  is  doubtful  whether  it  can  be  appealed 
to  in  support  of  the  view  that  in  other  cases  the  chromo- 
somes remain  intact. 


^& 


a 


b 


c 


FiQ.  22. — Sister  blastomeres  of  Ascaris  preparatory  to  another  division,  showing  similar 

arrangements  of  chromosomes.      (After  Boveri.) 

The  most  convincing  evidence  comes  from  exceptional 
cases  of  accidental  or  irregular  distribution  of  one  or 
more  chromosomes,  so  that  an  ^^^^  or  a  cell  comes  to  have 
one  more  chromosome  than  is  usually  present.  In  the 
thread-worm  Ascaris  there  are  two  varieties — one  that 
has  four  chromosomes  in  the  embryonic  cells  (with  two  as 
the  reduced  number)  and  another  variety  with  two 
chromosomes  (with  one  as  the  reduced  number).  A  few 
females  have  been  found  in  which  the  unfertilized  eggs 
contain  one  of  these  numbers,  and  all  of  the  spermatozoa 
that  have  been  received  from  another  individual  the 
other  number.     In  such  cases  the  fertilized  eggs,  and 


MECHANISM  IN  SEGREaATION  53 

all  embryonic  cells,  have  three  chromosomes  each  (Fig. 
64),  showing  that  when  an  egg  starts  with  three  chromo- 
somes, this  number  is  retained  through  all  subsequent 
divisions,  despite  the  fact  that  after  each  division  a  rest- 
ing stage  intervenes. 


\ 


^  9^\  • 


-  •'-•r«  **•  '*  *• 


;«• 


••i^.  ^,  'A    am  !»• 


•.'5^  •«•  .T:  f^®  ft^*-' 


"*.  ^J;.  i  •»■ 


c 


Fig.  23. — Normal  a,  b,  and  reduced  a',  b',  chromosomes  of  two  species  of  Biston;  and  of 

hybrid  c,  c'. 

The  evening  primrose,  CEnothera  Lamar ckiana,  has 
14  chromosomes  (reduced  number  7).  Individuals  are 
known  in  which  there  are  15  chromosomes.  As  a  result 
of  accidental  displacement  at  a  division  in  a  germ-cell, 
possibly  one  cell  came  to  contain  an  additional  chromo- 
some.   Such  a  cell  combining  with  a  normal  one,  at  fertil- 


54 


PHYSICAL  BASIS  OF  HEEEDITY 


ization,  would  produce  a  plant  of  the  15-cliromosonie 
type.  Here  again,  the  additional  chromosome  persists 
as  an  individual  element  of  the  cell  throughout  subsequent 
cell-srenerations. 


/  w" 


Wni^"  /l!   U  / 


f///iimi!liPHl\\       ^         fe^ 


/Wi^iiiiiii 


Fig.  24. — Lagging  and  elimination  of  chromosomes  in  hybrid  fish  embryos.    (After  Pinney) . 

In  Drosophila  a  female  occasionally  appears  with  two 
X's  and  a  Y-chromosome.  There  are  several  ways  in 
which  this  may  arise,  but  the  most  common  way  appar- 
ently is  for  an  egg  to  retain  both  of  its  X  elements.  Such 
an  egg  fertilized  by  a  Y-bearing  sperm  produces  an  XXY 
embryo.    Such  an  embryo  retains  throughout  the  entire 


MECHANISM  IN  SEGREaATION  55 

development  (cell-divisions)  its  two  X's  and  its  Y.  There 
is  evidence  for  this,  obtained  by  Bridges,  both  from 
observation  of  the  cells  themselves  and  from  the  genetic 
behavior  of  such  an  individual. 

In  certain  crosses  between  moths  with  different  num- 
bers and  sizes  of  chromosomes,  Federley,  and  Harrison, 
and  Doncaster  have  shown  that  the  cell  of  the  hybrid 
contains  half  the  number  of  each  species,  even  with 
their  characteristic  size  differences  (Fig.  23).  In  crosses 
between  different  species  of  fish,  where  the  size  differ- 
ences are  quite  conspicuous,  it  has  been  shown  by  Moenk- 
haus,  Morris  and  Pinney  (Fig.  24)  that  the  embryonic 
cells  may  continue  through  their  divisions  to  retain 
the  characteristic  chromosomes  of  both  species.  These 
hybrid  cases  are  particularly  significant ;  for  the  chromo- 
somes derived  from  the  father  are  in  the  foreign 
medium  of  the  protoplasm  of  the  other  species.  Never- 
theless, in  some  cases  they  retain  their  own  peculiarities, 
through  successive  cell  generations. 

Evidence  That  Homologous  Chromosomes  Mate  with 

Each  Other 

That  the  mating  of  the  chromosomes  in  pairs  is  not  a 
haphazard  process,  but  that  each  paternal  chromosome 


'c^% 


Fig.  25. — Female  and  male  chromosome  groups  of  Protenor.     (After  Wilson.) 


mates  with  a  definite  maternal  chromosome,  has  been 
established  by  evidence  from  several  sources.  In  many 
species  the  chromosomes  are  of  different  sizes,  and  some- 
times certain  ones  are  markedly  different  in  size  from  the 
others.     In  the  bug  Protenor  the  two  sex-chromosomes 


56 


PHYSICAL  BASIS  OF  HEEEDITY 


of  the  female  are  conspicuously  larger  than  the  others 
(Fig.  25,(2).     When  reduction  takes  place  the  sizes  of 


Maturation     Divisions    of     Protlnor  Q 


I 


■Sip 


Oocyte 


r  Polar  Spmdlc  2"^  Polar  Spindle 


Mature    C^^  and 
Polav  Bodies 


Fig.  26. — Reduced  chromoBome  group;   and  extrusion  of  polar  bodies  in  Proienor. 

Maturation       Divisions      or     Protenor      (5 

^1  '  1 1 


jf. 


OQ 


0 


OO 


Ifim 


^o  o 


^^    \id 


WQo 


Melaphase  oj  the  ^  j/^    ^   O      q 


1  ^'  Spermatocyte  -^/i^ 

Amapliase  ot  the 

P^  Spermalocyle  %C^9o 

Metapkseoj  the      ^  -^J/^" 

2^  Spermatocyte 

Anaphase  o|  the 

Fig.   27. — Reduced  chromosome  group  of  male;     and  spermatogenesis  in  Protenor. 

the  fused  jDairs  show  that  these  two  large  chromosomes 
must  always  unite  with  each  other  (Fig.  26).  In  the 
male  of  certain  species,  as  in  Protenor  (Fig.  27),  the 


MECHANISM  IN  SEGREQATION  57 

sex  chromosome  has  no  mate,  and  therefore  nothing  to 
fuse  with.  Its  size,  after  the  others  have  conjugated 
(Fig.  27)  shows  that  it  remains  single;  while  its  failure 
to  divide  twice,  as  do  the  other  chromosomes,  corrobo- 
rates the  view  that  having  no  mate  of  its  own  it  never 
combines  with  any  other.  At  the  other  extreme,  the  two 
very  minute  chromosomes  in  several  of  the  Drosophila 
species  must  have  united  to  form  the  smallest  chromo- 
some of  the  reduced  series  (Fig.  28,  a-a\  h-b').  In  a 
few  cases  the  X  and  the  Y  are  different  in  size.  Wlien 
they  fuse  (in  the  male)  the  size  of  the  fused  mass  is  what 


¥.?!^ 


0 


a  (t 


Fia.   28. — Diploid  and  haploid    chromosome   groups   of   Drosophila   lu^ckii,  a,  a',  and  D. 

melanica,  (neglecta)  b,  b'.     (After  Metz.) 

is  expected,  viz.,  the  sum  of  the  masses  of  X  and  Y,  and 
their  subsequent  separation  into  parts  corresponding  in 
size  to  the  fused  bodies  supports  the  view  that  conjuga- 
tion amongst  the  chromosomes  is  a  very  definite  process. 
In  the  very  exceptional  case  of  a  bug,  Metapodius,  there 
is  a  pair  of  small  chromosomes  called  m's.  When  the 
other  pairs  enter  the  spindle  the  two  m's  come  together, 
touch,  and  then  separate,  to  pass  to  opposite  poles. 

Resume 

The  evidence  from  studies  of  the  maturation  of  eggs 
and  sperm  shows  that  the  paternal  and  maternal  chromo- 


58  PHYSICAL  BASIS  OF  HEREDITY 

somes  come  together  at  this  time  in  pairs,  and  subse- 
quently separate,  so  that  each  egg  comes  to  contain 
one  or  the  other  member  of  a  pair.  The  same  process 
takes  place  in  the  formation  of  the  sperm-cells.  It  is 
obvious  that  if  one  member  of  any  pair  contains  material 
that  produces  an  effect  on  some  character  as  one  of  the 
end  results  of  its  activity,  and  the  other  member  of  the 
pair  contains  a  different  material,  the  behavior  of  the 
chromosomes  at  the  time  of  maturation  supplies  exactly 
the  mechanism  that  MendePs  law  of  segregation  calls  for. 


ii 


CHAPTER  IV 

MENDEL'S  SECOND  LAW— THE  INDEPENDENT 
ASSORTMENT  OF  THE  GENES 

Mendel  proved  that  when  races  differ  from  each 
other  in  two  pairs  of  characters,  each  pair  considered 
by  itself  alone  gives  the  3 : 1  ratio,  and  the  inheritance 
of  one  pair  is  independent  of  that  of  the  other.  If  a  tall 
race  of  peas  with  colored  flowers  is  crossed  to  a  short 
race  with  white  flow^ers  the  offspring  show  the  two  domi- 
nant characters,  i.e.,  they  are  tall  and  have  colored 
flowers.  If  these  are  inbred  they  produce  tall  and  short 
offspring  (Fo)  in  a  ratio  of  3: 1,  and  these  same  individ- 
uals, if  reclassified  for  pigment,  are  colored  or  white  in 
the  ratio  of  3 : 1.  For  example,  the  ideal  for  12  tall  peas 
would  be  9  colored  and  3  white;  and  for  4  short  peas 
there  would  be  3  colored  and  1  white.  Expressed  in  a 
diagram  we  have: 

12  tall  4  short 

9  colored   :   3  white  3  colored.   :   1  white 

The  preceding  w^ay  of  stating  the  results  deals 
directly  with  the  facts.  The  explanation  of  these  results, 
based  on  the  segregation  of  the  members  of  the  two 
independent  pairs  of  factors,  is  as  follows:  If  we  call 
the  gene  for  tallness  by  the  same  name  as  the  character 
itself,  viz.,  tall,  and  the  gene  for  shortness  by  the  name 
of  this  character,  viz.,  short,  and  similarly  for  the  other 
pair  of  characters,  viz.,  color  versus  white,  then  when 
crossed  the  hybrid  has  two  pairs  of  allelomorphs, 

tall      color 


short    white 

59 


60 


PHYSICAL  BASIS  OF  HEREDITY 


If  at  the  maturation  (whether  of  egg  or  sperm)  tall  and 
color  go  to  one  cell,  then  short  and  white  go  to  the  other 
cell;  but  if  one  of  the  pairs  is  turned,  so  to  speak,  the 
other  way,  thus 

short     color 


tall      white 

so  that  short  and  color  go  to  one  cell,  then  tall  and  white 
go  to  the  other.  Four  classes  of  germ-cells  are  expected 
in  i^i,  namely, 

tall  coior,       tall  white,       short  color,       short  white. 

Chance  meeting  of  any  one  of  these  four  kinds  of  pollen 
grains  with  any  one  of  the  same  four  kinds  of  eggs  will 
give  the  sixteen  recombination  classes  shown  in  the  fol- 
lowing table: 


Eggs 


Sperm 
tall  color 


tall  white 
short  color 
short  white 


tall  color 


tall  white 


short  color        short  white 


tall  color 
tall  color 

tall  white 
tall  color 

short  color 
tall  color 

short  white 
tall  color 

tall  color 
tall  white 

tall  white 
tall  white 

short  color 
tall  white 

short  white 
tall  white 

tall  color 
short  color 

tall  white 
short  color 

short  color 
short  color 

short  white 
short  color 

tall  color 
short  white 

tall  white 
short  white 

short  color 
short  white 

short  white 
short  white 

The  four  kinds  of  eggs  are  written  above  and  the  four 
kinds  of  sperm  are  written  to  the  left.  There  are  16  pos- 
sible combinations.  Since  tall  and  color  are  dominant 
the  recombinations  give :  9  tall  color,  3  tall  white,  3  short 
color,  1  short  white.  In  this  table  the  genes  have  the 
same  name  as  the  character  for  which  they  stand,  and 
these  names  are  written  out  in  full,  but  it  is  generally 
more  convenient  to  use  symbols  for  the  genes  in  order 


MENDEL'S  SECOND  LAW 


61 


to  save  space  and  time.  It  is  customary  to  represent 
the  members  of  a  pair  by  the  same  letter,  as  Mendel 
himself  did,  and  to  represent  the  dominant  member  by 
the  capital  letter,  the  recessive  member  by  a  small  letter. 
Thus  if  ^  =  tall  and  a  =  short;  and  5=^  color  and 
&  =  white,  the  recombination  square  becomes: 


Eggs         AB 


Sperm 
AB 


Ab 
aB 
ab 


Ab 


aB 


ab 


AB 
AB 

Ab 
AB 

aB 
AB 

ab 
AB 

AB 

Ab 

Ab 

Ab 

aB 
Ab 

ab 
Ab 

AB 
aB 

Ab 
aB 

aB 
aB 

ab 
aB 

AB 
ab 

Ab 
ab 

aB 

ab 

ab 
ab 

Instead  of  using  arbitrary  letters  for  the  characters 
as  above,  it  has  been  found  more  convenient  to  use  a 
mnemonic  system  in  which  the  first  letter  of  one  of  the 
members  of  each  pair  becomes  the  symbol.  The  two 
members  of  such  a  pair  are  then  distinguished  from  each 
other  by  using  a  capital  letter  for  one  and  a  correspond- 
ing small  letter  for  the  other.  For  example,  we  might 
let  t  =  short,  T  =  tall,  c  =  white,  C  =  color.  In  this 
case  the  capital  letter  represents  the  dominant  character, 
and  the  small  letter  represents  the  loss  of  that  character, 
as  seen  in  the  recessive  type.  But  besides  prejudging  the 
question  as  to  what  kind  of  a  change  took  place  in  the 
germ-plasm  to  change  a  dominant  to  a  recessive  by 
assuming  that  it  is  due  to  a  loss,  this  system  is  unsatis- 
factory in  cases  where  many  modifications  of  the  same 
organ  exist  (such  as  the  40  eye  colors  of  the  vinegar  fly), 


62 


PHYSICAL  BASIS  OF  HEREDITY 


and  where  new  ones  are  being  found.  For  example,  if  the 
symbol  R  (red)  is  used  for  the  dominant  wild  eye  color, 
small  r  would  stand  for  any  one  of  40  mutant  eye  colors, 
and  when  several  of  these  occur  in  the  same  experiment 
there  would  be  no  way  of  telling  for  which  one  the  small 
letter  stood.  Some  other  system  becomes  imperative 
in  such  cases, ^  and  the  most  consistent  seems  to  be  to 
use  a  small  letter  for  the  mutant  gene  in  question  (or 
when  unknown  for  the  recessive  gene),  and  the  corre- 
sponding capital  letter  for  its  allelomorph  (usually  the 
wild  type).  Thus,  5  =  short,  iS^^tall,  21;:=  white,  TF  = 
color.  The  recombination  square  for  the  same  characters 
treated  above  is  then: 


Eggs 

Sperm 
SW 


Sw 


sW 


sw 


SW 


Sw 


sW 


sw 


sw 
sw 

Sw 
SW 

sW 
SW 

sw 

sw 

sw 

Sw 

Sw 
Sw 

sW 

Sw 

sw 
Sw 

sw 

sW 

Sw 
sW 

sW 
sW 

sw 
sW 

sw 

sw 

Sw 
sw 

sW 

sw 

sw 
sw 

Since  the  large  letters  simply  represent  the  wild  type  of 
each  particular  character  it  may  sometimes  simplify  the 
formulae  to  omit  them,  or  since  this  may  lead  to  confusion 
in  making  up  pairs  of  genes,  some  convention  for  wild 
type,  such  as  N  (normal),  T  (type),  or  the  +  sign,  or  a 
dash,  or  a  dot  may  be  used.  Such  short-hand  methods 
are  followed  by  many  workers,  but  it  is  not  necessary  to 
advance  the  claims  of  any  one  of  them  here.     If,  for 


*  An  even  more  serious  objection  to  the  system  is  explained  in  "The 
Mechanism  of  Mendelian  Heredity, "  pages  233-235. 


MENDEL  ^S  SECOND  LAW 


63 


example,  the  normal,  meaning  the  wild  type  in  each  factor 
pair,  is  represented  by  N,  the  foregoing  table  becomes : 


Eggs 


»  Sperm 

NN 


Nw 
sN 

8W 


NN 


Nw 


sN 


8W 


NN 
NN 

Nw 

NN 

sN 
NN 

sw 

NN 

NN 
Nw 

Nw 
Nw 

sN 
Nw 

sw 
Nw 

NN 
sN 

Nw 

sN 

sN 

sN 

sw 

sN 

NN 
sw 

Nw 

8W 

sN 
sw 

sw 
sw 

In  the  preceding  illustration  one  grand-parent  (Pj)  was 
assumed  to  have  had  both  dominant  characters  (tall  and 
colored),  while  the  other  grand-parent  had  both  reces- 
sives  (short  and  white).  Obviously  the  grand-parents 
might  have  happened  to  be  made  up  differently — one 
might  have  been  tall  and  white,  the  other  short  and  col- 
ored. The  Fi  plants  {Ss,  Wiv)  would  have  been  the 
same  in  either  case,  and  so  would  the  F2  results.  In 
other  words,  for  the  principle  of  assortment  it  should 
make  no  difference  from  which  parent  the  characters  have 
come.  This  is  illustrated  in  the  following  cross  (Fig.  29), 
in  which  a  wingless  vestigial  (recessive)  Drosophila 
male  having  the  wild-type  color  (dominant)  is  bred  to 
long- winged  (dominant)  female  with  ebony  (recessive) 
body  color.  The  F^  flies  have  long  wings  and  wild  type 
body  color.  Inbred,  they  give  9  long  wild  type  color,  3 
long  ebony,  3  vestigial  wild  type  color,  and  1  vestigial 
ebony.  In  the  diagram  the  gene  for  vestigial  is  repre- 
sented by  V,  and  its  allelomorph  for  long  wings  by  V;  the 
gene  for  ebony  by  e,  its  allelomorph  for  wild  type  color 
by  E,  The  germ-cells  of  the  two  Pj  flies  are  therefore 
vE  and  Ve.  Each  contains  the  wild-type  allelomorph 
of  the  recessive  mutant  gene  in  the  other  parent.    The 


64  PHYSICAL  BASIS  OF  HEREDITY 

F^  fly  has  the  formula  vVeE.    Independent  assortment 
of  the  two  pairs  of  factors 

v_  E^ 

T  T 

gives   four  kinds   of  germ-cells   both  in  males   and  fe- 
males, thus : 

vE     Ve     VE    ve 

Any  one  of  the  four  kinds  of  egg  may  be  fertilized  by  any 
one  of  the  same  four  kinds  of  sperm  giving  the  same  result 
as  in  the  case  of  the  peas,  viz.,  four  kinds  of  F2  individuals 
in  the  ratios  of  9 :  3 :  3 : 1.  In  practical  tests  the  occur- 
rence of  or  the  possession  of  a  race  with  both  recessives 
in  it  is  highly  desirable  for  use  in  making  a  back-cross  to 
Pj  (instead  of  inbreeding  F^'s),  because  the  numerical 
results  obtained  by  back-crossing  furnish,  for  a  smaller 
number  of  individuals,  more  significant  data.  For  exam- 
ple, if  a  tall  pea  with  colored  flowers  is  crossed  to  a 
short  pea  with  white  flowers,  and  the  F^  individuals 
(SsWw)  are  back-crossed  to  short  white  peas  (sw),  the 
expected  ratio  will  be  1:1:1:1,  because  the  four  kinds 
of  gametes  in  Fj  (SW,  Sw,  sW,  sw)  will  then  reveal 
themselves  in  the  offspring,  since  the  double  recessive 
individual  (sw)  used  for  back-crossing  (having  only 
recessive  gametes)  will  not  ^^ cover  up''  any  of  the  factors 
coming  from  the  Fj  hybrid.  For  instance,  as  shown 
in  our  type  example,  the  Fj  gametes  are  SW,  Sw, 
sW,  sw.  The  only  kind  of  gamete  produced  by  the 
double  recessive,  short  white,  is  sw.  When  this  meets 
each  of  the  above  gametes  only  four  kinds  of  combina- 
tions are  possible,  vis^.,  SWsiv,  Swsw,  sWsw,  swsw;  and 
these  zygotes,  containing  only  the  same  dominants  as  the 
Fj  gametes,  will  reveal  what  the  kinds  of  gametes  were. 
In  practice  an  approximation  to  a  1 : 1 : 1 : 1  ratio  is  much 
more  likely  to  be  evident  than  an  approximation  to  a 
9 :  3 :  3 :  1  in  which  only  one  double  recessive  individual  out 


MENDEL'S  SECOND  LAW 


65 


[HD    CUD 
or:) 


CUD 

EH)    CUD 


31 


Canutits. 


Ft    Zjcpte    <S  flW  9 


Fia.  29. — Cross  between  wingless  and  ebony  vinegar  fly. 


66 


PHYSICAL  BASIS  OP  HEREDITY 


of  16  individuals  is  expected.  "Wlienever  possible,  there- 
fore, the  back-cross  experiment  is  preferable  to  the 
inbred  F^  cross. 

In  animals  and  in  plants  with  separate  sexes  it  has 
been  found  that  both  F^  males  and  F^  females  give  when 
back-crossed,  identically  the  same  results,  showing  that 
free  assortment  takes  place  in  both  sexes. 


FiQ.  30. — Miniature  wing,  a;  and  dumpy,  b;   and  minature  dumpy,  c. 

There  is  a  corollary  to  the  cross  involving  two  pairs 
of  factors  that  is  interesting,  because  it  gives  an  explana- 
tion of  the  phenomena  of  atavism.  The  wild  vinegar  fly, 
Drosophila  melanogaster  (Fig.  4),  has  long  wings.  It 
gave  rise,  through  mutation,  to  a  race  with  miniature 
{mm)  wings  (Fig.  30,  a),  and  also  to  another  race  with 
short  wings  (Fig.  30,  h)  called  ''dumpy*'  {dd).  If  a 
female  miniature  (mmDD)  is  crossed  to  a  dumpy  male 
(MMdd),  all  the  offspring  (MmDd)  have  long  wings  like 


MENDEL'S  SECOND  LAW  67 

those  of  the  wild  fly.  The  miniature  fly  carries  the  domi- 
nant {BD)  wild-type  allelomorph  of  the  dumpy  gene,  and 
the  dumpy  carries  the  dominant  {MM)  wild-type  allelo- 
morph of  the  miniature  gene.  Since  the  hybrid  contains 
the  two  wild-type  genes  {DM)  it  ^'reverts''  to  the  long- 
wdnged  fly.  The  proof  that  two  pairs  of  factors  are  in- 
volved is  found  by  inbreeding  an  F-^  male  and  female, 
which  give  9  long,  3  miniature,  3  dumpy,  and  1  miniature 
dumpy  (Fig.  30,  c)  fly. 

There  are  certain  modifications  of  the  two-pair  ratio 
that  arise  sometimes  when  different  factors  produce  a  like 
effect  on  the  same  organ.  Such  cases  have  sometimes  been 
treated  as  special  cases,  and  rather  peculiar  interpreta- 
tions given  to  them  on  the  basis  that  the  situation  is 
unusual.  In  reality  they  are  only  interesting  cases  of 
Mendelian  behavior,  the  results  obscured  to  some  degree 
by  superficial  character  relations.  The  absence  of 
color,  albinism,  is,  perhaps,  the  most  familiar  example 
of  this  sort.  There  are  certain  recessive  factors 
that  when  homozygous  interfere  in  some  unknown  way 
with  the  development  of  color.  Albinos  of  the  ordinary 
house  mouse  are  white  because  they  are  homozygous  for 
the  albino  factor,  although  they  may  be  pure  for  all  other 
factors  that  are  essential  for  color.  If  a  certain  kind  of 
albino  mouse  is  crossed  to  a  pure  black  mouse  the  off- 
spring will  be  gray  because  black  (&&),  being  recessive  to 
its  wild-type  allelomorph  {BB),  brought  in  by  the  albino, 
disappears;  and  white  {wiv)  being  recessive  to  its  wild 
type  allelomorph  for  color  {WW),  brought  in  by  the 
black,  also  disappears,  so  that  the  color  of  the  resulting 
animal,  gray,  is  due  to  the  hybrid  having  recovered  all 
the  factors  that  give  this  color.  The  two  factor-pairs 
involved  are  black  {h)  and  its  normal  allelomorph  {B  = 
gray),  and  white  {w)  and  its  normal  allelomorph 
(TF=color).  The  F2  results,  put  into  the  recombination 
square,  are  as  follows : 


68 


PHYSICAL  BASIS  OF  HEREDITY 


Eggs 
BW 


Bw 


bW 


bw 


BW 
BW 
gray 

Bw 
BW 

gray 

bW 

BW 

gray 

bw 

BW 

gray 

BW 

Bw 

gray 

Bw 

Bw 

white 

bW 
Bw 
gray 

bw 
Bw 
white 

BW 

bW 
gray 

Bw 
bW 
gray 

bW 

bW 

black 

bw 

bW 

black 

BW 

bw 
gray 

Bw 
bw 
white 

bW 

bw 

black 

bw 
bw 
white 

Sperm 
BW 

Bw 

bW 

bw 


The  resulting  ratio  is  9  grays,  3  blacks,  and  4  whites.    The 
Jast  two  terms  of  the  9:3:3:1  ratio  are  here  united  in  one 

Solas s  (4  whites )%ecause\vvhen  homozygous  for  absence  of 
color  the  individual  is  white,  regardless  as  to  whether 
the  other  color-producing  factors  make  for  the  wild  type 
of  coloration  or  for  some  mutant  color. 
"  Another  interesting  two-pair  case  involves  varieties 
of  the  combs  of  domesticated  breeds  of  fowls.  There  is  a 
dominant  type  called  ^^Rose^'  (Fig.  31,  c),  which,  bred  to 
single  (wild  type.  Fig.  31,  a),  gives  Rose  in  Fj,  and  3  Rose 
to  1  Single  in  Fg.  Another  dominant  type  called  ''Pea'' 
(Fig.  31,  b)  likewise  gives  Pea  in  F^  and  3  Peas  to  1  Single 
Comb  in  F2.  But  when  Rose  is  bred  to  Pea  there  is  not 
produced  the  wild  type,  as  one  might  have  anticipated,  but 
a  comb  called  ''Walnut"  (Fig.  31,^),  that  differs  from 
both  parental  types.  The  character  is  due  to  the  com- 
bined action  of  both  dominants.  If  two  F^  birds  with 
Walnut  combs  are  bred  to  each  other  they  give  9  Walnut, 
3  Pea,  3  Rose,  1  Single  comb.  This  ratio  shows  that  two 
factors  are  involved,  and  that  the  Walnut  comb  appears 
in  all  birds  carrying  both  the  Rose  and  the  Pea  genes. 
The  Single  comb  is  the  double  recessive  form. 


MENDEL'S  SECOND  LAW 


69 


If  the  single  comb  be  supposed  to  be  the  wild  type, 
then  Pea  and  Rose  represent  dominant  mutant  types 


•,U 


d 


Fig.  31. — Combs  of  fowls,  single,  a;  rose,  h;  pea,  c;  and  walnut,  d. 

Neither  produces  any  single  comb,  if  the  races  are  homo- 
zygous for  Pea  or  for  Rose  respectively,  but  when  crossed, 
the  Pea  comb  brings  in  the  normal  recessive  allelomorph 


70  PHYSICAL  BASIS  OF  HEKEDITY 

of  Rose,  and  the  Rose  comb  the  normal  recessive  allelo- 
morph of  Pea:  but  the  result  is  not  an  atavistic  normal 
comb,  but  a  Walnut  produced  by  the  action  of  both  domi- 
nants that  are  here  the  mutant  characters. 

An  important  class  of  factors  that  are  known  as 
diluters  or  intensifiers  are  often  met  with  in  genetic 
work.  For  instance,  a  black  mouse  pure  for  a  certain 
'^ diluting ^^  factor  has  a  '^blue"  color  (just  as  a  black 
mouse  pure  for  albino  factors  is  white).  Such  a  blue 
mouse  crossed  to  black  gives  F^  black  mice,  and  in  F2, 
three  blacks  to  one  blue.  A  two-factor  cross  results  when 
a  blue  mouse  is  bred  to  a  ^'chocolate"  (=black  cinnamon) 
mouse.  The  F,  will  be  black,  the  F^  will  be  9  black,  3  blue, 
3  chocolate,  1  ^^silver-fawn''  (dilute  black  cinnamon). 
In  this  case,  the  same  factor  that  changes  black  to  blue  also 
changes  chocolate  to  silver-fawn.  If  the  diluter  had 
been  a  specific  one  affecting  black  only,  then  Fo  from  the 
above  cross  would  have  been  9  black,  3  blue,  4  chocolate. 
Such  a  case  is  found  in  the  vinegar  fly,  in  which  the  diluter 
affects  only  a  recessive  factor — eosin.  This  specific 
diluter  for  eosin  is  called  * '  whiting. ' '  It  gives  the  follow- 
ing results :  A  red-eyed  female  homozygous  for  whiting 
is  indistinguishable  from  the  ordinary  wild  type.  If  a 
female  of  this  kind  is  crossed  to  an  eosin  male  the  off- 
spring (Fj)  are  red  eyed.  If  they  are  inbred  they  give  12 
red-eyed  flies,  3  eosin,  1  eosin-whiting  which  is  colorless. 

Another  modification  of  the  9:3:3:1  ratio  appears 
when  the  last  three  classes  are  superficially  alike.  For  ex- 
ample, Bateson  and  Punnett  crossed  two  white  flowering 
varieties  of  sweet  peas.  The  F^  had  purple  flowers,  w^hich, 
inbred,  gave  9  purple  and  7  whites.  Here  there  are  two 
different  recessive  factors  which  in  homozygous  condition 
give  white,  wiv  and  aa;  each  has  a  normal  dominant  allelo- 
morph in  the  other  white,  AA  and  WW.  The  two  white 
parents  are  then  wwAA  and  aaWW.    The  Fj  individuals 


MENDEL'S  SECOND  LAW 


71 


are  WwAa,  and  the  four  gametes  are  WA,  Wa,  wA,  wa. 
The  table  below  gives  the  sixteen  recombinations 
of  these  gametes: 


Eggs 


Sperm 
WA 


Wa 


wA 


wa 


WA 


Wa 


wA 


wa 


WA 

WA 

purple 

Wa 

WA 

purple 

wA 

WA 

purple 

wa 
WA 
purple 

WA 

Wa 

purple 

Wa 

Wa 

white 

wA 

Wa 

purple 

wa 
Wa 
white 

WA 

wA 
purple 

Wa 

wA 
purple 

wA 

wA 

white 

wa 
wA 
white 

WA 

wa 
purple 

Wa 

wa 
white 

wA 
wa 
white 

wa 
wa 
white 

Any  individual  that  has  both  recessives  ivw  or  aa  is  white. 
There  are  7  such  classes  to  9  that  carry  both  A  and  W. 
Lastly,  a  15 :1  modification  of  the  9:3:3:1  ratio  is  obtained 
when  an  individual  homozygous  for  both  pairs  of  recessive 
genes  gives  a  different  result  from  any  other  combina- 
tion. Thus,  Shull  found  when  Bursa  pastoris,  with  trian- 
gular capsules,  is  crossed  to  one  with  round  capsules,  the 
latter  appears  in  F2  only  once  in  16  times. 

ASSOBTMENT    OF    ThEEE    FaCTORS 

When  three  independent  factor-pairs  are  present  the 
numerical  expectation  can  be  directly  derived  from  the 
9:3:3:1  ratio  in  the  same  way  that  the  latter  was  derived 
from  the  3 : 1  ratio.     Thus : 


3 


27:9        9:3 


One  pair  of  factors. 
Two  pairs  of  factors. 
Three  pairs  of  factors. 


72  PHYSICAL  BASIS  OF  HEEEDITY 

Each  F2  class  of  the  two-factor  case  (9:3:3:1)  will  con- 
tain a  three-to-one  ratio  for  the  third  factor-pair.  Thus, 
in  the  9  class  there  will  be  3  dominants  of  the  third  factor 
to  one  recessive  (27:9).  So  for  each  3  class:  each  con- 
tains the  third  factor  in  the  ratio  of  3:1.  So  also  for 
the  1  class.    The  total  result  therefore  is : 

In  actual  practice  the  three-factor  cases  are  almost 
never  used.  Other  methods  are  employed  to  detect  the 
factors  present,  so  that  these  three-factor  ratios  have  a 
theoretical  rather  than  a  practical  value.  In  cases  where 
multiple  factors  are  suspected,  some  of  them  may  be  only 
modifiers  of  some  one  of  the  other  more  conspicuous  char- 
acters and  in  such  cases  special  methods  of  procedure  will 
recommend  themselves. 


CHAPTER  V 
THE  MECHANISM  OF  ASSORTMENT 

Each  pair  of  chromosomes,  just  before  the  reduction 
division,  consists  of  a  maternal  and  a  paternal  member. 
As  the  members  of  each  pair  are  in  nearly  all  cases  identi- 
cal in  appearance,  it  is  not  possible  to  tell  how  they  place 
themselves  on  the  mitotic  spindle  with  respect  to  their 
parental  origin ;  that  is,  it  is  not  possible  to  tell  by  inspec- 
tion whether  at  the  maturation  division  all  those  of  mater- 
nal origin  pass  to  one  pole  of  the  spindle,  and  all  those  of 
paternal  origin  to  the  other,  or  whether  the  pairs  come  to 
lie  haphazard  on  the  spindle,  so  that  it  is  merely  a  matter 
of  chance  whether  a  maternal  or  a  paternal  member  passes 
to  a  particular  pole.  For  the  utilization  of  the  chromo- 
somal mechanism  for  the  theory  of  assortment,  it  is  a 
matter  of  great  importance  which  of  the  preceding  alter- 
natives is  followed,  for  if  all  of  the  maternal  chromosomes 
should  go  to  one  pole,  and  all  the  paternal  to  the  other, 
there  would  be  no  free  assortment  of  the  chromosomes, 
and  no  free  assortment  of  the  genes  if  these  are  carried  by 
the  chromosomes.  Without  random  assortment  there 
could  only  be  two  kinds  of  gametes  produced  by  the 
hybrid,  hence  only  three  types  possible  in  F2,  viz.,  the  two 
grandparental  types  and  the  hybrid  type. 

On  the  other  hand,  if  the  assortment  of  chromosomes 
is  a  random  one,  then  the  reduction  division  furnishes  the 
mechanism  that  MendePs  law  calls  for  in  so  far  as  the 
character-pairs  lie  in  different  chromosome  pairs. 

There  is  not  a  single  cytological  fact  opposed  to  the 
view  of  free  assortment  of  maternal  and  paternal  chromo- 
somes; on  the  contrary,  there  is  a  general  expectation 
that  the  chromosomes  should  assort  freely;  and  what  is 
more  to  the  point,  there  are  a  few  crucial  cases  that  show 
that  free  assortment  takes  place.    Let  us  turn  to  these 

73 


74  PHYSICAL  BASIS  OF  HEREDITY 

cases.  The  most  convincing  evidence  is  that  furnished 
by  Miss  Carothers  (1913)  from  some  grasshoppers  of  the 
genus  Brachystola.  Here,  in  addition  to  the  single  sex- 
chromosome  (in  the  male),  that  goes  to  one  pole  of  the 
first  maturation  spindle,  there  is  also  present  another  pair 
of  chromosomes  that  are  unequal.  In  some  cells  the 
smaller  member  of  the  pair  goes  to  the  same  pole  as  the 
sex-chromosome,  in  other  cells  it  goes  to  the  opposite 
pole.  The  assortment  of  the  unequal  pair  as  regards  the 
sex-chromosome  is  tlierefore  a  random  one.  Thus,  in 
three  hundred  first  spermatocytes,  the  smaller  partner 
went  to  the  same  pole  as  the  sex-chromosome  in  48.7  per 
cent,  of  cases,  and  into  the  cell  without  the  sex-chromo- 
some in  51.3  per  cent.  Voinov  ('14),  Wenrich  ('14)  and 
Robertson  ('15)  have  reported  similar  cases. 

Other  evidence  of  a  different  kind  has  more  recently 
('17)  been  described  by  Miss  Carothers.  The  evidence 
rests  on  the  constancy  of  attachment  of  the  fibres  of  the 
mitotic  figure  to  a  definite  point  of  the  chromosome,  as 
seen  when  the  chromosomes  are  moving  towards  the  poles 
of  the  spindle.  In  one  of  the  cases  she  describes  there 
are  two  kinds  of  attachments,  viz.,  terminal,  when  the  fibre 
is  attached  at  the  end  of  the  rod-shaped  chromosome,  and 
suhterminal  when  the  fibre  is  attached  some  distance  from 
the  end.  In  the  latter  case  the  end  bends  over,  making 
the  chromosome  J-shaped.  There  are  certain  individuals 
in  which  one  member  of  a  pair  of  chromosomes  may  have 
a  terminally  attached  fibre,  and  its  mate  have  a  subter- 
minally  attached  fibre.  Throughout  all  the  cell-divisions 
of  such  an  individual  these  two  chromosomes  show  this 
difference.  During  maturation,  i.e.,  after  conjugation  of 
the  chromosomes,  one  member  of  this  pair  passes  to  the 
pole  of  the  spindle  with  a  terminal  attachment,  and  its 
mate  with  a  subterminal  to  the  other  pole.  In  the  male,  the 
single  sex-chromosome  passes  to  one  or  to  the  other  pole 
at  one  spermatocyte  division.  Its  relation  to  the  two 
members  of  the  pair  of  chromosomes  in  question  will  show 


THE  MECHANISM  OF  ASSORTMENT        75 

whether  random  assortment  or  correlated  movement  takes 
place.  Observation  shows  that  sometimes  one,  sometimes 
the  other,  member  of  the  pair  goes  to  the  same  pole  as  the 
sex-chromosomes. 

It  happens  that  in  a  species  studied  by  Miss  Carothers 
{T rimer otro pis  suffiisa)  there  are  several  chromosomes 
that  may  show  constantly  terminal  or  subterminal  attach- 
ment of  the  fibres;  as  many  as  seven  out  of  the  twelve 
chromosomes  of  the  first  spermatocyte  division  may  con- 
sistently show  this  difference.  In  other  words,  any  one  of 
these  seven  chromosomes  may  have  one  or  the  other  kind 
of  attachment.  Each  grasshopper  may  have  any  one 
of  ten  of  its  pairs  showing  combinations  of  these  kinds  of 
attachment,  but  of  course  in  any  one  individual  only  two 
possible  arrangements  exist  for  a  given  pair  of  chromo- 
somes. It  is  to  be  remembered  that  for  a  given  combina- 
tion all  the  cells  of  an  individual  are  exactly  alike,  which 
incidentally  is  a  strong  argument  in  favor  of  the  individ- 
uality of  the  chromosomes.  An  example  will  give  further 
details.  In  Fig.  32  are  shown  eight  groups  of  chromo- 
somes {h,  c,  d,  e,  f,  g,  h,  j)  from  the  same  individual.  Each 
group  of  12  chromosomes  comes  from  a  single  cell  about 
to  divide.  Each  series  of  12  is  here  arranged  in  a  single 
horizontal  line.  The  dividing  chromosome  is  a  tetrad,  one 
of  whose  halves  is  about  to  separate.  It  is  significant  to 
note  that  in  this  case  the  separating  halves  represent  the 
two  conjugating  members  of  each  pair;  in  other  words, 
the  reduction  division  is  taking  place.  In  this  individual, 
four  of  the  tetrads  (9-12)  has  subterminal  attachment 
only,  i.e.,  for  both  members  of  the  dividing  pair  (dyad) ; 
four  of  the  tetrads  (Nos.  2,  3,  5  and  6)  have  terminal 
attachments  only,  while  the  remaining  three  tetrads  (Nos. 
1,  7  and  8)  have  one  end  with  a  terminal  attachment,  and 
the  other  subterminal.  In  addition  there  is  the  sex- 
chromosome  (No.  4)  that  is  here  going  upwards  toward 
the  top  of  the  figure,  and  will  pass  with  the  upper  half 
of  each  tetrad  into  an  imaginary  cell  above  (the  female- 


76  PHYSICAL  BASIS  OF  HEREDITY 

producing  sperm).  The  first  double  chromosome  (No.  1) 
has  a  different  mode  of  fibre  attachment  to  each  half, 
but  the  halves  are  here  not  different  in  size.  In 
five  cases  the  chromosome  with  terminal  attachment  is 
going  to  the  cell  that  will  get  the  sex-chromosome  (the 
upper  one  here),  while  in  three  cases  it  goes  to  the  pole 
that  will  not  get  the  sex-chromosome.  Chromosomes  7 
and  8  are  slightly  different  in  size,  but  this  is  not  distin- 
guishable in  these  figures.  In  the  first  four  cells  {viz., 
h,  c,  d,  e)  the  halves  of  7  and  8  with  subterminal  attach- 
ments are  going  to  opposite  poles ;  in  the  remaining  four 
cells  (fy  g,  }i,  i)  they  are  going  to  the  same  pole.  Again,  if 
we  compare  Nos.  7  and  8  with  No.  1  it  is  found  that  in  four 
cells  (/,  g,  h,  i)  the  half  with  terminal  attachment  passes 
into  the  cell  with  the  same  attachment  (/  and  i)  (for  7 
and  8),  and  the  other  half  into  the  cell  with  the  other 
attachment  (g  and  h).  In  other  words,  the  distribution 
for  four  chromosomes  pairs  (1,  4,  7,  8)  is  here  a  random 
assortment.  Let  A,B,C  represent  the  chromosomes  with 
one  kind  of  attachment,  and  a,  h,  c  their  mates  with  the 
other  kind  of  attachment.  B  is  the  sex-chromosome  and 
d  its  absence.  There  will  then  be  sixteen  possible  assort- 
ments of  these  four,  all  equally  probable.    Thus: 

ABCD     aBCB     ahCB  ahcB     ahcd 

AhCB    AhcB  ahCd 

ABcB     ABcd  aBcd 

ABCd     aBcB  Ahcd 

aBCd 

AhCd 

There  were  100  spermatocytes  recorded  by  Miss  Carothers 
as  to  the  distribution  of  their  chromosomes  to  the  two 
poles,  giving  data  for  200  cells.  Their  distribution  as 
well  as  the  expectation  for  free  assortment  is  as  follows : 

Expected        Realized 
Only  one  chromosome  with  sub-terminal  attachment.  %  X  200=50       48 
Any  two  chromosomes  with  sub-terminal  attachment.  Yi  X  200=75       84 
Any  three  chromosomes  with  sub-terminal  attachment. X   X  200  =50       48 
Any  four  chromosomes  with  sub-terminal  attachment.  yV  X  200  =  123--^     8 


THE  MECHANISM  OF  ASSOETMENT        77 
12        «l       10       9       8       7        6       5      4      3     2     I 

I'^iG.   32, — Eight  chromosome  groups  of  twelve  chromosomes  each  of  Trimerotropis.    (After 

C^arothers.) 


78  PHYSICAL  BASIS  OF  HEREDITY 

For  a  CO  ant  of  only  100  cells  the  agreement  with  expecta- 
tion is  sufficiently  close  to  show  that  independent  assort- 
ment takes  place. 

In  addition  to  the  differences  of  attachment  just  exam- 
ined there  are  other  differences  that  Miss  Carothers  has 
studied.  A  constriction  is  found  in  certain  chromosomes 
in  some  individuals  (Fig.  32,  No.  5)  that  is  absent 
in  other  individuals.  In  some  individuals  the  tetrad  is 
separating  so  that  the  group  looks  like  four  beads  in  a  line. 
In  other  cases  one  member  of  the  pair  is  not  constricted, 
while  its  mate  is  constricted.  Similarly  for  another 
chromosome.  In  one  individual  both  halves  of  the  dyad 
show  a  constriction,  while  another  individual  has  one 
smooth  and  one  constricted  half.  These  same  two  kinds 
of  chromosomes  also  have  in  some  cases  terminal  attach- 
ment, and  in  other  cases  subterminal,  making  possible 
further  combinations  that  can  be  identified. 

Finally,  there  are  two  types  of  subterminal  attach- 
ment in  two  chromosomes  of  the  series.  In  one  type  the 
chromosome  is  bent  further  from  the  end  than  in  the  other. 
Either  of  these  two  types  may  have  a  mate  of  the  other 
type  with  terminal  attachment,  thus  giving  several  fur- 
ther identifiable  combinations.  ^^All  possible  combina- 
tions of  the  dyads  in  these  two  types  of  heteromorphic 
tetrads  occur  and  segregate  [assort]  freely  in  relation 
to  sex. ' '  Miss  Carothers  points  out  that  when  three  types 
of  the  same  chromosome  exist  ''we  have  a  visible  mechan- 
ism whose  behavior  in  the  maturation  divisions  corre- 
sponds to  the  segregation  of  triple  allelomorphs.'* 

In  addition  to  the  12  ordinary  chromosomes  certain 
individuals  may  have  a  small  thirteenth  or  even  a  four- 
teenth chromosome.  These  are  called  supernumeraries. 
In  Circotettix  they  were  found  present  in  two  of  eleven 
individuals  examined.  If  present,  it,  or  they,  are  constant 
for  all  cells  except  that  at  the  reduction  division  there  may 
be  a  new  distribution.  If  one  is  present  it  may  go  to 
either  pole  with  reference  to  the  sex-chromosome,  and  at 


THE  MECHANISM  OF  ASSORTMENT         79 

the  second  spermatocyte  division  it  divides  as  do  the  others 
at  this  time  in  the  cell  that  contains  it.  If  two  are  present 
they  do  not  behave  as  mates,  but  at  the  first  spermatocyte 
division  may  both  go  to  the  same  pole  (which  may  be 
either  pole  in  reference  to  the  sex-chromosome),  or  they 
may  go  to  opposite  poles.  At  the  second  spermatocyte 
division  each  divides  independently,  and  halves  go  to 
opposite  poles.  These  bodies  then  also  move  to  either 
pole  without  respect  to  other  chromosomes — or  at  least 
without  respect  to  the  sex-chromosomes ;  but  this  behavior 
can  scarcely  be  used  to  advantage  for  the  question  of 
assortment  because  these  chromosomes  have  no  mates  (in 
the  cases  so  far  described)  and  are  so  inconstant  in  their 
occurrence  that  an  appeal  to  their  behavior  as  bearing 
on  the  other  chromosomes  might  not  be  conceded.  If  they 
are  pieces  of  other  chromosomes  (the  bent  ends,  for  exam- 
ple) that  have  been  broken  off,  we  might  expect  them  to 
show  some  relation  during  synapsis  to  the  original 
chromosome  from  which  they  came,  but  as  yet  nothing 
of  the  sort  has  been  described.  If  they  carry  factors  that 
influence  the  characters  of  the  individual,  their  presence, 
especially  when  two  occur,  would  give  rise  to  unexpected 
genetic  results. 

The  evidence  furnished  by  cytolog}^  that  has  just  been 
given  makes  clear  that  whenever  an  opportunity  has  been 
found  to  study  the  mode  of  assortment  of  the  chromosomes 
the  result  shows  random  distribution.  If  then  the  chromo- 
somes carry  the  genes  for  the  hereditary  characters,  we 
should  expect  that  the  genes  in  different  chromosome  pairs 
will  ^*  assort  ^^  independently,  and  this,  in  fact,  is  what 
Mendel's  second  law  postulates. 


CHAPTER    VI 
LINKAGE 

Mendel  ^s  results  involving  two  or  more  pairs  of  char- 
acters led  to  the  conclusion  that  distribution  of  the  mem- 
bers of  one  pair  of  genes  is  independent  of  the  distribution 
of  the  members  of  other  pairs.  This  process  may  be 
called  free  or  independent  assortment,  and  is  what  is 
expected  if  each  pair  of  genes  is  carried  by  a  different 
pair  of  chromosomes.  If  this  rule  held  for  all  pairs  of 
characters  then  there  could  be  no  more  pairs  that  assorted 
independently  than  there  were  pairs  of  homologous 
chromosomes.  On  the  other  hand,  if  the  chromosomes 
carry  the  genes  we  should  anticipate  from  what  we  have 
found  out  concerning  the  individuality  of  the  chromo- 
some, and  from  what  we  know  concerning  the  large 
number  of  inherited  characters,  that  many  of  these  fac- 
tors must  be  carried  in  the  same  chromosome.  If  this 
is  true,  then  Mendel's  second  law  can  have  only  a  very 
limited  application. 

As  our  information  about  the  mode  of  inheritance  of 
characters  has  widened,  the  number  of  cases  in  which  free  4Sllf 
assortment  does  not  occur  has  steadily  increased.  Many 
characters  have  been  found  to  keep  together  in  successive 
generations.  This  tendency  to  keep  together  rather  than 
to  assort  freely  is  called  linkage.  The  most  extreme  cases 
are  those  where  characters  hold  together  completely; 
at  the  other  extreme  are  those  that  show  only  a  slightly 
greater  probability  of  holding  together  than  of  assorting 
freely.  Between  these  extremes  all  intermediate  degrees 
of  linkage  are  found.  For  the  sake  of  simplicity,  cases  of 
complete  linkage  will  be  dealt  with  in  this  chapter;  the 
others  will  be  taken  up  in  the  next  chapter. 

If  a  fly  (Drosophilii)  with  two  recessive  mutant  charac- 
80  • 


LINKAGE 


81 


ters,  black  body  color  and  vestigial  wings  (Fig.  33),  is 
mated  to  a  fly  with  wild-type  body  color  and  long  wings, 
the  offspring  (F,)  are  wild  type.  If  one  of  the  F,  sons 
is  back-crossed  to  a  black  vestigial  female  from  stock, 


Fig.  33. — Back-cross  of  Fi  male  (out  of  black  vestigial  by  wild),  to  black  vestigial. 

the  offspring  {F2)  are  of  two  kinds  only,  half  are  black 
vestigial,  and  the  other  half  are  wild  type.  In  other 
words,  the  two  mutant  characters  that  went  in  together, 
black  and  vestigial,  have  come  out  together ;  and  their  two 

6 


82  PHYSICAL  BASIS  OF  HEREDITY 

normal  allelomorpbic  characters,  wild-type  body  color  and 
long  wings,  have  also  come  out  together.  There  are  no  Fo 
flies  that  are  black  and  long,  and  none  that  are  vestigial 
and  gray,  as  would  be  the  case  if  independent  assortment 
took  place. 

In  the  diagram  (Fig.  33)  the  results  are  worked  out  on 
the  chromosome  theory.  The  genes  for  black  (b)  and  for 
vestigial  (v)  are  represented  as  carried  by  the  same 
chromosome  (hv) ;  the  homologous  chromosome  of  the 
wild-type  fly  carries  the  normal  allelomorpbic  genes  (BV), 
In  Fj,  one  of  each  of  these  two  chromosomes  is  present, 
and  the  fly  is  normal  because  the  two  normal  allelomorphs 
are  dominant.  In  the  Fi  male  these  two  chromosomes 
(hv  and  BV)  separate  at  the  reduction  division  of  the 
germ-cells,  one  going  to  each  gamete.  If  this  F^  male  is 
mated  to  a  black  vestigial  female,  all  of  whose  eggs  carry 
genes  for  black  and  for  vestigial,  the  offspring  should 
reveal  the  composition  of  the  gametes  of  the  F^  male,  since 
the  eggs  of  the  black  vestigial  fly,  containing  only  two 
recessive  factors,  will  not  cover  up  the  effects  of  the  fac- 
tors contained  in  the  gametes  of  the  Fi  male. 

Unless  we  knew  that  the  two  characters  black  and 
vestigial  are  distinct  mutant  characters,  the  preceding 
experiment  would  not  necessarily  show  that  the  char- 
acters are  linked,  because  the  same  result  would  have 
followed  if  black  and  vestigial  were  both  due  to  the  effect 
of  a  single  gene.  Other  experiments,  however,  show  that 
they  are  independent  characters. 

It  is  interesting  to  compare  the  preceding  cross  with 
another  in  which  black  comes  in  from  one  parent,  and  ves- 
tigial from  the  other.  For  instance,  if  a  black  fly  with 
long  wings  is  crossed  to  a  wild-type  fly  with  vestigial  wings 
(Fig.  34),  the  F^  offspring  will  be  wild  type  both  in  their 
color  and  in  their  wings,  because  the  black  fly  brings  in 
the  normal  allelomorph  of  vestigial,  and  the  vestigial  fly 
brings  in  the  normal  allelomorph  of  black.  If  the  F^ 
sons  are  back-crossed  to  black  vestigial  females,  the  off- 


I 


LINKAGE 


83 


spring  are  of  two  kinds  only,  namely,  black  long,  and  wild- 
type  color  vestigial.  The  combinations  that  went  into 
the  cross  together  have  come  out  together.    The  diagram, 


Fig.  34. — Back-cross  of  Fi  male  (out  of  gray  vestigial  by  black)  to  black  vostijjial. 

based  on  the  chromosomes,  shows  that  the  genetic  results, 
as  before,  follow  the  chromosome  behavior,  provided  there 
has  been  no  interchange  of  genes  in  the  male. 

For  the  sake  of  simplicity  only  two  linked  factors  werr- 


84  PPIYSICAL  BASIS  OF  HEEEDITY 

utilized  in  the  preceding  cases.  Three,  four,  five,  or, 
theoretically,  any  number  of  characters  may  show  this 
relation  to  each  other.  Thus  there  is  a  stock  of  Droso- 
phila  with  five  linked  mutant  characters,  namely,  black, 
purple,  curved,  plexus,  speck.  In  a  back-cross,  like  the 
one  above,  all  the  mutant  characters,  if  they  went  in 
together,  will  come  out  together  in  half  of  the  second 
generation  (back-cross)  flies,  and  their  wild  type  allelo- 
morphic  characters  in  the  other  half. 

There  is  another  way  in  which  linkage  may  be  very 
simply  illustrated.     There  are  certain  characters,  called 


Diploid  Nuclei       XX 


Gametes        .    X  X       Y 

Fertilization 
Zygotes 

Fig.  35. — Scheme  showing  the  inheritance  of  the  sex-chromosome  in  Drosopkila. 

sex-linked  characters,  because  their  factors  follow  the  sex- 
chromosomes,  or  may  be  said  to  be  carried  by  them  or  to 
be  in  them.  Now  in  Drosophila,  the  female  has  two  X- 
chromosomes  (Fig.  35),  the  malo  one  X  (and  a  Y).  After 
reduction  the  eggs  have  each  one  X  chromosome.  Any 
such  egg  fertilized  by  a  F-bearing  sperm  will  produce  a 
male  (XY),  as  shown  in  the  scheme  below.  The  single  X- 
chromosome  that  this  male  gets  is  therefore  from  his 
mother.  If  her  X-chromosome  carried  sex-linked  factors, 
these  should  be  present  in  the  son.  Such,  in  fact, is  the  case. 
For  example,  a  female  Drosophila  with  yellow  wings  and 
white  eyes  mated  to  a  wild-type  male  will  produce  wild- 
type    females,    and    yellow    white-eyed    sons    (like    the 


LINKAGE  85 

mother) .  Here  the  son  gets  his  sex-linked  characters  from 
his  mother,  since  his  only  X  is  derived  from  her.  Experi- 
ments have  shown  that  this  holds  for  any  number  of 
sex-linked  characters  that  are  present  in  the  mother.* 

Linkage  has  been  demonstrated  in  a  number  of  animals 
and  plants.  The  first  case  discovered  was  in  sweet  peas. 
Bateson  and  Punnet t  (1905)  found  that  when  purple 
flowers  and  long  pollen  grains  went  in  from  one  parent, 
and  red  flowers  and  round  pollen  went  in  from  the  other 
parent,  they  tended  to  come  out  together  more  frequently 
than  would  be  expected  on  the  two-factor  ratio,  9:3:3:1. 
In  the  case  of  these  sweet  peas  the  linkage  is  not  com- 
plete, apparently  not  in  either  sex.  At  present  two  dif- 
ferent linkage  groups  are  known  in  sweet  peas,  one  made 
up  of  three  linked  characters,  and  the  other  of  three,  pos- 
sibly four.  In  the  edible  or  garden  pea  there  are  two 
linked  characters,  and  two  that  are  doubtful  (Bateson 
and  Vilmorin,  White).  Mendel  did  not  happen  to  make 
any  combinations  of  linked  characters  in  this  form,  hence 
he  got  free  assortment.  In  the  primrose  (P.  sinen- 
sis) there  is  a  group  of  five  linked  characters  (Gregory, 
Altenburg) ;  in  the  snapdragon  a  group  of  five  (Baur) ; 
in  stocks  a  group  of  three  or  four  (Saunders).  In  the 
groundsel  (Senecio  vulgaris)  there  are  two  linked  charac- 
ters known;  other  cases  occur  in  corn  (Lindstrom), 
tomatoes  (Jones),  wheat  (Engledow),  oats  (Surface), 
Oenothera  (DeVries,  Muller).  In  animals  the  largest  num- 
ber of  linked  characters  is  found  in  the  vinegar  fly,  Droso- 
phila  melanogaster,  in  which  there  are  four  groups — a 
sex-linked  group  containing  about  100  characters,  a  second 
group  containing  75  characters,  and  a  third  group  contain- 
ing about  60  characters,  and  a  fourth  group  of  two  charac- 
ters. In  other  species  of  Drosophila,  linked  characters 
(other  than  sex-linked)  are  beginning  to  be  reported  as 
more  characters  are  studied  (Metz  in  D.  virilis,  Warren 

*  A  reservation  for  crossing  over  in  a  heterozygous  mother  must  be 
added  to  this  statement. 


S6  PHYSICAL  BASIS  OF  HEREDITY 

in  D.  huschii,  Sturtevant  in  D.  repleta).  Nabours  has 
found  a  case  in  one  of  the  grouse  locusts,  and  Castle  and 
Wright  in  rats.  In  the  silk-worm  moth,  Tanaka  has  found 
one  group  of  linked  characters.  In  poultry  Goodale  has 
found  one  case.  In  the  moths  and  poultry  it  appears 
that  linkage  is  complete  in  the  female,  incomplete  in  the 
male.  In  this  respect  the  situation  is  the  reverse  of 
that  in  Drosophila.  There  are  some  other  cases  where 
linkage  is  suspected  but  uncertain. 

The  fact  that  relatively  so  few  cases  of  linkage  have 
been  as  yet  reported  is  due  in  part  to  the  fact  that  in 
most  species  the  heredity  of  only  a  very  few  charac- 
ters is  generally  known.  Where  more  are  known  each 
has  as  a  rule  not  been  examined  in  relation  to  all  the 
others,  so  that  even  if  some  of  the  factors  were  linked  it 
would  not  have  been  found  out.  Furthermore,  in  Mendel- 
ian  crosses,  the  practice  of  mating  F^^s  instead  of  back- 
crossing,  tends  to  conceal  the  linkage  phenomena  if  pres- 
ent. The  fact  of  greatest  significance,  however,  is  that 
the  number  of  cases  of  linkage  is  steadily  increasing  as 
the  inheritance  of  more  characters  in  each  species  is 
becoming  known. 


CHAPTER    VII 
CROSSING  OVER 

The  correlative  aspect  of  linkage  is  crossing  over,  and 
inasmuch  as  it  involves  a  change  in  the  mechanism  that 
gives  linkage,  it  is  entitled  to  rank  as  one  of  the  funda- 
mental principles  of  heredity. 

In  the  illustration  of  complete  linkage  given  in  the 
preceding  chapter,  the  cases  chosen  were  ones  in  which 
entire  chains  of  genes  remained  intact  during  the  reduc- 
tion divisions.  The  male  of  Drosophila  exhibits  this  phe- 
nomenon, as  does  also  the  female  of  the  silk-worm  moth. 
On  the  other  hand,  there  is  an  interchange  of  blocks  of 
genes  between  homologous  pairs  of  chromosomes  in  other 
cases,  as  in  the  females  of  Drosophila,  in  the  males  of 
moths  and  fowls,  and  in  both  egg-cells  and  sperm-cells 
of  the  plant  Primula}  This  interchange  is  called  crossing 
over,  and  the  evidence  shows  that  it  is  not  haphazard,  but 
gives  numerical  results  of  extraordinary  constancy.  A 
few  examples  will  suffice  to  illustrate  crossing  over. 

When  a  black  fly  with  vestigial  wings  is  crossed  to 
a  wild-type  (''gray")  fly  with  long  wings  (Fig.  33)  the 
offspring  are,  as  we  have  already  seen,  ''gray,"  long. 
If  one  of  the  F^  females  is  back-crossed  to  a  black  ves- 
tigial male  there  are  four  kinds  of  offspring  produced,  viz., 
the  two  original  combinations,  black  vestigial,  and  gray 
long;  and  in  addition  two  recombinations  of  these,  viz.y 
black  long,  and  gray  vestigial.  The  two  latter  classes  arc 
called  the  crossover  classes,  or  more  briefly,  crossovers. 
The  percentage  of  crossovers  is  definite  for  a  given  stock, 

^  Crossing  over  in  both  sexes  in  tlie  rat  has  been  reported  by  Castle 
and  Wright,  and  in  the  male  and  female  grasshopper  by  Naboura. 

87 


88 


PHYSICAL  BASIS  OF  HEREDITY 


of  a  given  age  and  under  given  environmental  conditions. 
In  this  case  the  percentages  are  as  follows : 

Crossovers 
Black  long  Gray  vestigial 

s  A  r».ov  cent  8.5  per  cent 


Non-crossovers 
Black  vestigial  Gray  long 

41.5  per  cent  41.5  per  cent 

83  per  cent 


8.5  p 


17  per  cent 


If  a  pair  of  chromosomes  in  the  F^  fly  is  represented  as 
carrying  the  genes  of  the  characters  here  involved,  one 
member  of  such  a  pair  carries  both  a  gene  for  black  and 
a  gene  for  vestigial  (Fig.  36) ;  the  homologous  member 
of  the  pair  of  chromosomes  carries  both  of  the  normal 
allelomorphs,  viz.,  a  gene  for  gray  and  a  gene  for  long 
wings.  When  crossing  over  takes  place  so  that  a  gene 
for  black  goes  over  into  the  other  chromosome,  the  con- 
verse phenomenon  takes  23lace,  a  gene  for  gray  goes  over 
into  the  chromosome  that  gave  up  its  black  gene.  It  is  the 
constancy  of  this  interchange  that  makes  the  phenomenon 
reducible  to  exact  mechanical  treatment. 

The  interchange  is  independent  of  the  way  in  which  the 
genes  enter  the  cross.  For  example,  if  a  black  long- 
winged  fly  is  crossed  to  a  gray  vestigial  fly  (Fig.  37),  the 
Fi  offspring  will  be,  as  before,  gray  long.  If  an  F^ 
female  (gray  long)  is  back-crossed  to  a  black  vestigial 
male,  there  will  be  four  kinds  of  offspring,  namely,  the  two 
original  combinations  black  long,  and  gray  vestigial; 
and  the  two  crossover  combinations,  black  vestigial,  and 
gray  long,  in  the  following  proportions : 


Non-crossovers 
Black  long  Gray  vestigial 

41.5  per  cent  41.5  per  cent. 

• . ' 

83  per  cent 


Crossovers 
Black  vestigial 
8.5  per  cent  ' 


Gra}^  long 
8.5  per  cent 


17  per  cent 


The  interchange  in  the  last  cases  is  in  the  reverse  order 
of  that  in  the  first  case,  but  it  is  numerically  identical. 
In  other  words,  it  makes  no  ditference  whether  the  gene 
for  black  and  for  vestigial  enter  the  cross  together,  i.e.. 


CROSSING  OVER 


89 


Fig.  36. — Back-cross  of  Fi  female  (out  of  black  vestigial  by  wald)  to  black  vestigial  male. 

in  the  same  chromosome,  or  whether  they  enter  the  cross 
in  opposite  chromosomes — their  likelihood  of  interchange 
is  exactly  the  same.  If  the  Fj^  male  had  been  back-crossed 
(Fig.  34)  only  two  kinds  of  otf spring  would  have  been 


90 


PHYSICAL  BASIS  OF  HEREDITY 


Fio.  37. — Back-cross  of  F2  female  (out  of  gray  vestigial  by  black)  to  black  vestigial  male. 

produced  because,  as  was  shown,  there  is  no  crossing 
over  in  the  male. 

It  should  be  pointed  out  here,  that  the  interchange  (or 
crossing  over)  can  of  course  only  be  recorded  when  two 


CROSSING  OVER 


91 


or  more  pairs  are  involved,  for,  obviously,  unless  a  char- 
acter that  enters  the  cross  comes  in  with  some  other  known 
one  that  is  recognizable  as  such,  there  is  no  way  of  de- 
termining whether  interchange  between  the  homologous 
chromosomes  has  taken  place.  As  will  be  pointed  out 
later,  there  is  every  reason  to  suppose,  and  practically 
a  demonstration  of  the  fact,  that  the  interchange  goes  on 
irrespective  of  the  presence  of  other  genes  by  whicli  it 
can  be  observed. 

Experiments  with  different  pairs  of  characters  show 
that  for  each  two  pairs  there  is  a  definite  numerical  ratio. 
For  instance,  if  a  female  fly  with  yellow  wings  and  white 
eyes  is  crossed  to  a  fly  with  gray  wings  and  red  eyes 
(wild  type)  the  daughters  will  have  gray  wings  and 
red  eyes  (wild  type).  If  the  F^  female  is  back-crossed  to 
a  male  with  yellow  wings  and  white  eyes,  four  classes  of 
oif spring  will  be  produced  in  the  following  proportions : 


Non-crossovers 
Yellow  white  Gray  red 

49.5  per  cent  49.5  per  cent 

99  per  cent 


Crossovers 

Yellow  red  Gray  white 

0.5  per  cent  0.5  per  cent 

1  per  cent 


Here,  crossing  over  takes  place  in  only  one  case  out  of 
a  hundred.  If  the  characters  enter  in  a  different  combina- 
tion, viz,f  yellow  red  and  gray  white,  the  crossover  per- 
centage is  the  same  as  before,  viz., 


Non-crossovers 

Yellow  red  Gray  white 

49.5  per  cent  49.5  per  cent 

99  per  cent 


Crossovers 
Yellow  white  Gray  red 

0.5  per  cent  0.5  per  cent 

1  per  cent 


Another  combination  of  white  eyes  with  a  different  charac- 
ter shows  a  different  linkage.  If  a  female  fly  with  white 
eyes  and  miniature  wings  is  crossed  to  a  male  with  red 
eyes  and  long  wings  (wild  type),  the  F^  daughters  will 
have  red  eyes  and  long  wings.    If  one  of  these  F^  females 


92 


PHYSICAL  BASIS  OF  HEREDITY 


is  back-crossed  to  a  white  miniature  male  the  four  classes 
of  offspring  appear  in  the  following  proportions : 


Non-crossovers 
White  miniature  Red  long 

33.5  per  cent  33  5  per  cent 

67  per  cent 


Crossovers 
White  long  Red  miniature 

1G.5  per  cent  16.5  per  cent 

33  per  cent 


Here  crossing  over  takes  place  in  33  out  of  100  flies, 
whereas  in  the  former  crosses  between  white  eyes  and 
another  mutant  character  (yellow)  crossing  over  took 
place  only  once  in  a  hundred  times.  Based  on  these 
numerically  different  ratios  of  crossing  over,  and  on  other 
related  phenomena,  a  theory  of  crossing  over  has  been 
formulated  that  will  be  discussed  later.  For  the  present 
we  are  concerned  only  with  the  data. 

When  more  than  two  pairs  of  characters  are  involved 
new  phenomena  of  crossing  over  make  themselves  evident. 
Some  of  these  are  more  related  to  principles  that  are  dis- 
cussed in  later  chapters,  but  a  few  results  may  be  pointed 
out  here.  When,  for  example,  three  pairs  are  involved, 
two  may  cross  over,  while  the  third  does  not.  A  female 
with  white  eye  color,  miniature  wings,  and  forked  bristles 
crossed  to  a  wild-type  male  gives  wild  type  in  F^.  An 
Fj  daughter  back-crossed  to  a  white  miniature  forked  male 
will  give,  in  the  next  generation,  not  only  the  two  original 
combinations  but  recombinations  also.  As  we  have  seen, 
33  per  cent,  of  all  the  offspring  will  be  crossovers  between 
white  and  miniature;  there  will  also  be  20  per  cent,  of 
crossing  over  between  miniature  and  forked.  In  other 
words,  there  will  be  both  red  miniature  and  white  long 
flies,  and  there  wall  also  be  crossovers  between  white  and 
miniature,  i.e.,  miniature  wings  straight  spines,  and  long 
wings  forked  spines.  It  follows  that  there  may  also  be 
cases  in  which  crossing  over  has  taken  place  between  both 
of  the  above  combinations  at  the  same  time  (Fig.  38), 
that  is,  there  will  be  some  flies  that  are  white  long- winged 
and  forked  and  others  that  are  red  miniature  and  straight 


CEOSSING  OVER 


93 


spines.    A  list  of  these  classes  with  the  expectation  based 
on  the  results  from  a  single  experiment  is  given  below. 


Non-crossover 
wmf     23.2 

W  M  F2S.2 


Double  crossover 
in  both   I  "^  ^/  3.3 


regions 


W  m  F3.3 


Single  crossover 

in  1st     (  ^  ^^  ^  13-2 

^^g^^^  ]w  mf   13.2 

in  2nd   f  ^  ^"  ^     ^-^ 
region  \^,  j^jj   6  7 


Inasmuch  as  this  subject  and  certain  peculiarities  in 
the  results  can  be  better  understood  after  the  evidence  for 


w 


M 


/ 


/ 


Fig. 38. — Scheme  to  illustrate  double  crossing  over  between  white  and  forked.     The  gene 
for  miniature  standing  between  furnishes  the  evidence. 

the  linear  order  of  the  genes  has  been  discussed,  I  shall 
not  press  further  the  discussion  here.  It  should  be  pointed 
out,  however,  that  the  question  of  crossing  over  involves 
more  than  the  independent  action  of  the  pairs  in  the  cases 
so  far  considered;  for,  as  will  be  shown  later,  when  cross- 
ing over  takes  place  not  single  genes  but  great  groups  of 
genes  are  involved.    Tliis  block  effect,  as  it  may  be  called, 


94  PHYSICAL  BASIS  OF  HEREDITY 

is  not  in  evidence  unless  a  larger  number  of  genes  than 
two  is  studied  in  the  same  experiment.  These  questions 
will  be  discussed  in  Chapter  IX. 

For  the  purpose  of  clearer  exposition  I  spoke  of  link- 
age, in  the  preceding  chapter,  as  though  the  term  should 
be  limited  to  cases  where  all  the  genes  of  a  group  hold 
together,  and  have  used  the  tenn  crossing  over  to  mean 
the  breaking  of  the  group  in  one  or  more  pairs.  As  a 
matter  of  fact,  it  is  not  desirable  to  emphasize  this  sharp 
distinction.  There  is,  however,  a  real  distinction  that  lies 
behind  the  phenomena.  In  the  male  of  DrosopMla  there 
is  no  crossing  over  at  all  between  homologous  chromosome 
groups,  while  in  the  female  there  is  crossing  over  between 
the  pairs  of  chromosomes.  The  cases  of  the  male  and 
female  are,  therefore,  on  a  different  footing  here. 

We  speak  of  pairs  of  characters  as  being  loosely  linked, 
meaning  that  crossing  over  of  genes  frequently  takes 
place,  and  as  strongly  linked  when  crossing  over  is  very 
infrequent.  We  have  seen  that  yellow  and  white  break 
apart  only  once  in  100  times.  If  characters  (or  the  genes) 
were  still  more  closely  linked,  they  might  break  only  once 
in  a  thousand  times,  and  if  closer  still  once  in  many 
thousand  times,  in  which  case  they  would  appear  to  be  com- 
pletely linked  for  all  practical  purposes.  Such  a  grada- 
tion, however,  does  not  appear  to  be  the  case,  but  the  lower 
limit  of  crossing  over  seems  to  be  well  within  the  range  of 
human  capacity  to  detect.  This  means  probably  that 
there  is  a  limiting  value  for  crossing  over,  and  if  this 
can  be  established  it  may  give  us  the  lower  limit  of  size 
of  the  gene  (in  terms  of  chromosome  length),  or  at  least 
it  may  allow  us  to  form  some  idea  as  to  how  many  genes 
are  present  in  the  hereditary  material. 

In  this  same  connection  it  has  been  suggested  that 
when  more  than  two  allelomorphs  occur,  we  may  be  deal- 
ing only  with  close  or  even  absolute  linkage.  For  instance, 
suppose  in  a  white-eyed  race  of  flies  a  mutation  should 
take  place  in  a  gene  so  closely  tied  up  in  some  way  to  the 


m 


CROSSING  OVER  95 

gene  for  white  that  the  two  never  separated,  and  suppose 
the  new  mutation  affected  the  eye  so  that  its  effect  could 
be  observed  (for  if  not  the  change  would  not  concern  us). 
The  new  mutation  would  behave  towards  white  in  the 
same  way  as  do  all  pairs  of  allelomorphs  and  yet  in  a 
strict  sense  is  not  allelomorphic.  It  is  not  necessary  to 
elaborate  here  this  idea,  because  fortunately  in  the  case 
of  Drosophila  there  is  strong  evidence  to  show  that  multi- 
ple allelomorphs  do  not  arise  in  this  way.  The  evidence 
for  this  statement  will  be  given  in  Chapter  XVII. 


CHAPTER  VIII 
CROSSING  OVER  AND  CHROMOSOMES 

There  are  several  occasions  in  the  maturation  period 
of  the  germ-cells  when  it  would  seem  that  there  might  be 
an  opportunity  for  an  interchange  between  like  chromo- 
somes. Such  an  occasion  might  be  found  at  the  time 
when  the  thin  threads  twist  around  each  other;  or  it 
might  be  found  after  fusion  of  the  threads,  or  possibly 
after  a  general  breaking  up  of  the  chromosomes  and 
reunion  of  the  pieces.  Unfortunately  the  cytological 
evidence  does  not  furnish  explicit  information  as  to  the 
stage  at  which  interchange  takes  place. 

It  has  also  been  suggested  that  crossing  over  may  take 
place  at  a  still  earlier  stage  in  the  germ-tract,  i.e.,  long 
before  the  time  of  maturation,  even  in  the  early  embryo. 
Fortunately,  it  has  been  possible  to  obtain  critical  genetic 
evidence  showing  approximately  the  time  when  crossing 
over  takes  place.  This  evidence  was  obtained  by  Plough  in 
his  work  on  the  influence  of  temperature  on  crossing  over 
in  Drosophila  melanogaster. 

The  way  in  which  Plough's  experiment  was  carried 
out  was  as  follows :  Females  homozygous  for  three  mutant 
factors  in  the  second  chromosome,  viz.^  black,  purple, 
curved,  were  mated  to  wild-type  flies.  Some  of  these 
females  were  kept  in  an  incubator,  some  in  an  ice-box, 
and  some  were  kept  at  room  temperature;  under  one 
or  the  other  of  these  conditions  they  laid  their  eggs 
which  hatched  and  produced  larvae  and  pupae  and  flies.  The 
daughters  were  then  mated  to  black,  purple,  curved  males, 
and  remained  under  the  same  temperature  conditions  until 
their  offspring  hatched.  It  was  found  that  there  was 
more  crossing  over  in  the  offspring  of  the  pairs  kept  at  a 
high  and  at  a  low  temperature  than  in  those  kept  at  room 

96 


CROSSING  OVER  AND  CHROMOSOMES      97 

temperature.  Later  the  crossing  over  values  for  inter- 
mediate points  was  also  obtained,  and  from  these  data 
the  curve  shown  in  Fig.  56  was  made. 

At  a  low  temperature  (about  10°  C.)  crossing  over  is 
increased  as  compared  with  a  somewhat  higher  tempera- 
ture (18-27°  C).  Room  temperature  (22°  C.)  lies  in  that 
part  of  the  curve  where  there  is  the  least  amount  of  cross- 
ing over.  The  amount  then  rises  suddenly  until  about  29° 
and  remains  high  till  31°  C.  is  reached.  The  apparent 
faU  after  this  temperature,  as  shown  by  the  curve,  may 
not  be  significant.  The  flies  fail  to  lay  eggs  or  may  die 
at  about  this  point. 

In  the  foregoing  experiment  the  eggs,  larvaB  and  adult 
flies  had  been  kept  continuously  at  the  same  temperatures. 
If,  however,  the  heterozygous  virgin  females  reared  at 
high  temperature  are  back-crossed  to  the  triple  recessive 
males,  and  kept  afterwards  at  normal  temperature  (22° 
C.)  it  is  found  that  only  the  first  ten-day  output  of  such 
females  shows  the  high  crossing  over  values.  The  value 
drops  during  the  following  ten  days.  If  a  correction 
is  made  for  a  change  in  the  crossing-over  value  due 
to  age — since  age,  as  Bridges  has  shown,  causes  a  lower- 
ing in  the  value — still  the  effect  of  the  early  period  is 
found  to  have  begun  to  disappear  after  ten  days,  and 
soon  completely  disappears. 

In  still  another  way  the  influence  of  temperature  may 
be  shown.  Heterozygous  females  that  had  lived  at  nor- 
mal temperatures  are  mated  to  triple  recessive  males,  and 
then  exposed  for  the  first  seven  days  to  31.5°  C.  At  first 
the  normal  crossover  values  are  found,  as  seen  on  com- 
paring Fig.  40  mth  Fig.  39  which  is  the  control.  The 
latter  drops  slightly  from  the  second  to  the  eleventh  day. 
About  the  eighth  day  the  heat  effects  begin  to  show  (Fig. 
40),  and  there  is  a  sudden  and  considerable  rise  in  the 
curve,  that  lasts  for  ten  days,  when  it  drops  back  to  normal, 
corresponding  with  the  removal  of  the  flies  from  the  high 
to  normal  temperature,  i.e.,  after  the  seven-day  exposure. 
7 


98 


PHYSICAL  BASIS  OF  HEREDITY 


From  data  of  this  kind  it  is  possible  to  locate  the  stage 
in  the  development  of  the  egg  when  the  heat  is  affecting  it. 
If,  for  instance,  we  know  how  long  after  subjecting  a 


Percentage  of 
crossing  over 

2I[ 
19 


17 
16 

13 

II 

9 

7 
6 
S 


1 


J~T 


■^  Purple-curved 


^-lJ 


-  BiacK-purpie 


8      10      \2      14     16     18    20    22    24    26    20    30    32    34    36 


Days 


after  mating 


Fig.  39. — Curve  showing  the  influence  of  temperature  on  crossing  over;  control 

(After  Plough.) 


Percentage  of 
crossing  over 

84 


84 

22 

20 
18 
16 
14 

12 

10 
8 

6 


Purple-curved 


Black-purpie 


Days  after  mating 


2      4      6      8      10     12     14     16     18    20    22    24    26    28    30    32    34    36 
FiQ.  40. — Curve  showing  the  influence  of  temperature  on  crossing  over.      (After  Plough.) 

female  to  a  high  temperature,  the  effect  of  heat  on  crossing 
over  begins  to  be  observed  in  her  offspring,  and  also  how 
many  eggs  are  laid  by  the  female  before  this  influence 
is  manifest,  we  can  tell  approximately  in  what  stages  heat 


CEOSSING  OVER  AND  CHROMOSOMES      99 

affects  crossing  over.  Furthermore,  if  we  keep  eggs, 
larvae  and  pupae  in  a  high  temperature,  and  then  find  out 
how  many  eggs  have  been  affected  by  the  high  tempera- 
ture, we  can  find  out  to  what  stage  the  eggs  must  have 
developed  in  order  that  crossing  over  may  be  influenced. 
Plough  has  made  this  calculation,  and  finds  that  only  the 
eggs  that  have  reached  the  stage  where  conjugation  of 
the  chromosomes  takes  place  are  affected — all  the  earlier 
stages  are  not  influenced.  It  follows  that  the  initial  effect 
appears  at  about  the  time  of  conjugation  of  the  chromo- 
somes, but  whether  the  crossing  over  occurs  at  this  critical 
stage  or  some  effect  only  is  then  produced  that  later 
affects  the  crossing  over  is  not  specifically  shown.  Never- 
theless, I  am  inclined  to  think  it  more  probable  that  the 
crossing  over  is  actually  changed  at  the  time  the  heat  acts 
(rather  than  afterwards),  because  in  general  most  reac- 
tions of  living  things  to  environmental  influence  take  place 
immediately  rather  than  after  a  long  interval.  However 
this  may  be,  the  fact  of  prime  importance  in  this  work 
is  that  earlier  than  the  period  of  conjugation  of  the 
chromosomes  crossing  over  does  not  take  place. 

Expressed  in  numbers  of  eggs,  the  results  show"  that  in 
a  just-hatched  virgin  female  there  are  from  125  to  175 
eggs  that  will  be  laid  before  the  effects  of  heat  are  shown. 
In  the  females  that  have  just  hatched  about  150  eggs  are 
present  that  have  passed  beyond  the  conjugation  period. 
This  number  (150)  agrees  with  the  estimated  number  of 
eggs  (125-175)  first  laid  that  are  not  affected,  and  estab- 
lishes the  conclusion  that  after  conjugation  of  the  chromo- 
somes crossing  over  cannot  be  influenced  any  more  than 
it  could  before  that  period.  The  results  clearly  establish, 
then,  that  crossing  over  cannot  be  affected  earlier  than  the 
conjugation,  but  can  be  affected  at  the  time  when  the 
conjugation  is  known  to  occur. 

As  already  pointed  out,  the  chromosomes  become 
drawn  out  into  long  threads  at  the  synaptic  period,  and 
in  many  animals  and  plants  these  threads  have  been  shown 


100  PHYSICAL  BASIS  OF  HEEEDITY 

to  place  themselves  at  this  time  in  pairs.  The  double 
threads  shorten  later  to  take  on  the  form  of  the  ordinary 
chromosome.  How  the  earlier,  long  thin  thread  (lepto- 
tene  thread)  is  changed  into  a  thick  thread  when  the 
chromosomes  condense  is  not  known.  According  to  sev- 
eral accounts  the  thread  coils  spirally  within  the  wall  of 
the  ' '  chromosome, ' '  at  first  in  a  loose  coil,  then  in  a  tightly 
twisted  coil.  This  idea  of  a  coiled  thread,  or  core,  in  a 
condensed  chromosome  is  one  that  fits  in  very  well  with 
the  idea  that  the  thin  thread  represents  the  essential  ele- 
ment in  the  chromosome  that  retains  its  original  continuity 
even  when  the  chromosome  is  condensed  into  a  short  rod 
or  even  into  a  ball.  Unfortunately  the  evidence  in  favor 
of  this  view  is  by  no  means  well  established. 

At  the  time  when  the  threads  conjugate,  the  evidence 
in  several  forms,  such  as  Batracoceps,  Tomopteris,  etc., 
shows  that  when  the  conjugating  pairs  are  U-shaped,  the 
union  begins  apparently  at  both  ends  of  the  U  at  the  same 
time.  When  the  chromosomes  are  rod-shaped  (in  the  last 
telophase)  the  evidence  fails  to  show  whether  the  union 
begins  at  both  ends  simultaneously  or  at  one  end  only. 

As  the  union  between  the  threads  progresses  the  parts 
not  yet  united  can  often  be  seen  to  be  twisted  about  each 
other.  They  not  only  overlap,  but  they  seem  to  be  wrapped 
around  each  other. 

Whether  the  threads  are  split  lengthwise  before  their 
union  can  not  be  stated  for  all  cases.  It  is  certain  that 
no  splitting  has  been  seen  in  several  animals,  but  in  one 
case  (Ascaris)  the  threads  have  been  found  to  be  split 
lengthwise  before  they  conjugate. 

For  a  short  time  following  the  union  of  the  threads 
they  come  in  close  contact  with  each  other,  and  give  the 
impression  of  having  fused  into  a  single  thread.  Usually 
before  the  nuclear  wall  breaks  down  to  release  the  thick 
threads,  a  split  can  be  seen  again  extending  throughout 
the  length  of  the  threads.  Not  infrequently  another 
longitudinal  split  appears  in  each  half  resulting  from  the 


CROSSING  OYER  AND  CHROMOSOMES     101 

first  split,  so  that  four  parallel  strands  appear.  It  is 
customary  to  call  the  split,  that  is  supposed  to  correspond 
to  the  line  of  union  of  the  maternal  and  paternal  chromo- 
some, the  primary  split  or  reductional  split,  and  the  split 
that  corresponds  to  the  longitudinal  division  within  the 


Fia.  41. — Diagram  showing  crossing  over  of  two  chromosomes  at  the  four-strand  stage, 
a,  b,  and  the  subsequent  opening  out  of  the  tetrad,  d. 

maternal  or  the  paternal  chromosomes,  the  secondar}^  or 
equational  split.  Only  in  very  special  cases  is  it  possible 
to  be  able  to  say  which  is  the  primary  and  which  is  the 
secondary  split.  In  fact,  whenever  crossing  over  takes 
place  in  the  four-strand  stage  this  distinction  fails  to 
have  much  meaning. 


102 


PHYSICAL  BASIS  OF  HEEEDITY 


There  are  certain  questions  connected  with  crossing 
over  that  are  illustrated  by  the  following  models  (Figs. 
41,  42,  43) .  In  these  models  of  tetrads  the  dotted  rod,  split 
lengthwise,  stands  for  a  maternal  chromosome,  and  each 
of  its  halves  may  be  called  a  strand.  The  split  in  the  rod 
is  the  secondary  (or  equational)  split.  The  black  rod, 
also  split  lengthwise,  stands  for  the  paternal  chromosome. 

In  Fig.  41,  Qy  the  two  split  rods  are  represented  as 
twisted  about  each  other.  If  the  two  inner  strands  break 
and  the  cords  interchange  at  the  levels,  where  they  first 
come  into  contact  with  each  other  (Fig.  41,  Z>),  and  then 


FiQ.  42. — Scheme  showing  the  opening  out  of  the  strands  of  the  tetrad,  a,  in  two  planes, 

h,  according  to  Robertson  and  W  enrich. 


later  the  four  strands  come  to  lie  side  by  side,  i.e.,  ^'fuse," 
the  result  will  be  that  shown  in  Fig.  41,  c.  Two  of 
the  strands  represent  crossovers  in  the  sense  that  an 
interchange  has  taken  place  between  a  maternal  and  a 
paternal  strand ;  and  if  at  the  first  spermatocyte  division, 
when  the  threads  begin  to  pull  apart,  the  maternal  sepa- 
rate from  the  paternal  threads,  two  threads  may  be  seen 
actually  crossing  each  other  (Fig.  41,  d).  They  are  here 
the  two  non-crossover  strands,  but  if  the  two  strands 
thrown  to  the  left  had  been  thrown  to  the  right  the  two 
crossover  strands  would  cross  over.  The  scheme  is  essen- 
tiallv  the  same  as  the  chiasma  of  Janssens,  but  the  strands 
that  cross  may  or  may  not  (as  here)  represent  the  cross- 
over strands. 


CROSSING  OVER  AND  CHROMOSOMES     103 

The  next  two  figures  (Pig.  42,  a,  h)  show  how  Robert- 
son and  Wenrieh  interpret  the  crossed  threads,  that  they 
have  observed  in  the  spermatogenesis  of  some  of  the 
grasshoppers.  The  four  strands  are  represented  as  con- 
jugating side  by  side  in  Fig.  42,  a.  Wlien  the  strands 
begin  to  open  out  preparatory  to  the  first  spermatocyte 
division  the  two  maternal  separate  from  the  paternal  at  the 
ends  of  the  tetrad,  while  in  the  middle  of  the  tetrad  the 
opening  up  involves  the  separation  of  a  maternal  and  a 
paternal  strand  from  a  maternal  and  a  paternal.  In 
other  words,  the  tetrad  opens  up  in  two  planes  at  right 
angles  to  each  other.     This  scheme  also  gives  an  appa- 


ymmfm'm«>'^ffmfm?immmmmif!imt^m<m^mmif 


6 

Fiu.  43. — Scheme  showing  crossing  over  involving  both  strands  of  each  chromosome. 

rent  crossing  of  the  strands  at  the  level  where  the  open- 
ing out  in  one  plane  passes  over  into  the  opening  out 
in  the  other  plane,  but  there  has  been  no  real  crossing 
over  of  the  strands  in  the  sense  of  interchange  between 
them.  Theoretically  this  explanation  is  sound,  and 
moreover  seems  to  be  supported  by  observations  in 
cases  where  the  maternal  and  the  paternal  strands  can 
be  identified.  The  results  undoubtedly  show  that  the 
occurrence  of  crossed  threads  in  cases  where  the  split 
occurs  in  two  planes  does  not  necessarily  imply  that 
crossing  over  has  taken  place ;  but,  on  the  other  hand,  as 
has  been  shown  (in  Fig.  41)  a  similar  figure  may  also 
necessarily  result  after  crossing  over  of  the  threads.  In 
a  word,  the  crossed-strand  stage  is  not  ipso  facto  evidence 
that  it  must  have  come  about  according  to  Robertson  ^s 


104  PHYSICAL  BASIS  OF  HEREDITY 

sclieine.  It  should  also  be  observed  that  the  scheme  rests 
on  the  assumption  that  no  twisting  has  preceded  the  stage 
of  the  crossed  threads,  or,  if  such  has  taken  place  it 
has  no  relation  to  the  resulting  chiasma.  Yet  crossing 
of  the  threads  is  an  observed  fact. 

A  third  scheme  (Fig.  43,  a,  h)  makes  both  maternal 
strands  interchange  with  both  the  paternal  ones.  This 
scheme  has  at  least  one  formal  advantage  over  the  other 
two  in  that  it  represents  the  four  strands,  after  crossing 
over,  as  in  position  to  lie  side  by  side  in  the  tetrad,  so  that 
the  two  longitudinal  splits  that  reappear  later  lie  in  the 
same  plane  throughout  their  length.  This  seems  more  in 
accord  with  many  of  the  observations  that  are  recorded. 
If,  during  the  following  stages,  the  tetrads  open  out  by  the 
separation  of  the  maternal  from  the  paternal  strands  the 
crossed  threads  that  result  correspond  to  those  in  the  first 
scheme  (Fig.  41).  At  present  it  is  not  possible  to  decide 
between  these  ditferent  modes  of  representing  crossing 
over.  They  may  all  occur.  Their  discussion  shows  little 
more  than  certain  possibilities  involved  in  the  situation. 

Details  of  Spermatogenesis 

Some  of  the  stages  in  the  spermatogenesis  of  a  grass- 
hopper, Phrynotettix,  as  described  by  Wenrich,  are  shown 
in  the  following  figures.  The  material  furnishes  certain 
details  concerning  the  '^ resting  stages''  of  the  nuclei 
preceding  synapsis  more  completely  than  any  other,  and 
it  serves  also  to  illustrate  clearly  the  relationship  of  the 
chromosomes  to  the  vesicles  into  which  they  pass  (or 
which  they  form)  during  the  resting  stages.  The  figures 
also  show  how  the  threads  emerge  from  the  vesicles  in 
which  they  appear  to  have  been  contained  during  the 
resting  stages,  and  how  the  opening  out  of  the  tetrads 
in  two  planes  gives  the  appearance  of  chiasma  accord- 
ing to  Wenrich. 

Duiing  the  time  when  the  germ-cells  are  increasing  in 
number  by  division  there  is  a  resting  stage  after  each 


CROSSING  OVER  AND  CHROMOSOMES     105 

division  during  which  the  chromosomes  expand  into  a 
sort  of  vesicle,  as  seen  by  comparing  Figs.  44,  a  and 
44,  b.  An  optical  cross  section  of  the  stages  shown 
in  the  last  figure  is  represented  in  Fig.  44,  c.  An 
older  stage  is  seen  in  Fig.  44,  d.  The  stage  of  greatest 
diffusion  of  the  chromatin  material  within  its  vesicle  is 
seen  in  this  figure,  where  the  outlines  of  each  vesicle  are 
still  visible.    As  the  nucleus  gets  ready  for  another  divi- 


Fig.  44. — Spermatagonial  cells  in  the  last  phase  of  division,  a,  and  the  following  resting 

stages,  b,  d.     (After  Wenrich.) 

sion  the  vesicles  become  more  distinct  (Fig.  45,  a,h),  and 
soon  a  coiled  thread  can  be  seen  to  be  present  in  each 
vesicle  (Fig.  45,  c).  As  the  thread  thickens  (Fig.  45,  ^), 
a  longitudinal  split  appears  in  it,  which  indicates  the  plane 
of  division  of  each  chromosome  at  the  next  division. 

At  the  last  spermatogonial  division,  the  chromosomes 
of  the  two  daughter  nuclei  form  vesicles,  as  they  have  done 
in  earlier  divisions  (Fig.  46,  a  and  h).  But  changes  begin 
to  take  place  that  carry  the  chromosomes  through  a  very 
different  series  of  stages  from  those  seen  in  preparations 


106 


PHYSICAL  BASIS  OF  HEREDITY 


Fig.  45. — Cells  emerging  from  the  resting  stages  preparatory  for  the  next  spermatagonial 

division.      (After  Wenrich.) 


Vui.   46. — Cells  emerging  from  their  last  spermatagonial  division,    a.  b\  passing   into  the 

synapsis  stage,  r,  d;     (After  Wenrich.) 


CROSSING  OVER  AND  CHROMOSOMES     107 

for  the  ordinary  spermatogonial  (or  somatic)  cell-divi- 
sions. Each  chromosome  vesicle  begins  to  show  a  coiled 
thread  (Fig.  46,  c).  Each  thread  next  becomes  longer 
and  longer  (Fig.  46,  d)  until  the  whole  nucleus  is  filled 
with  them.  One  or  both  ends  can  often  be  seen  at  the 
^'distal  pole"  of  the  cell,  where  deep-staining  nucleoli 
are  present.  The  cells  are  now  in  the  so-called  thin 
thread,  or  leptotene  stages. 

The  threads  next  come  together  in  pairs  beginning 
at  the  distal  end  of  the  chromosomes  (the  zygotene  stage, 


Fig.  47. — Formation  of  a  thick  thread  after  synapsis,  a,  b;  and  the  following  condensation 

of  a  tetrad,  c.     (After  Wenrich.) 

Fig.  47,  a).  When  the  fusion  is  complete  and  all  the 
threads  are  double  (Fig.  47,  5),  the  stage  is  called  the 
thick  thread  or  pachytene  stage.  There  are  half  as  many 
threads  now  present  as  at  the  beginning.  A  longitudinal 
split  is  present  in  the  chromosome  throughout  these  stages 
along  the  line  of  fusion  of  the  two  thin  threads.  Wenrich 
identifies  the  split  as  the  "primary  split." 

Another  longitudinal  split  at  right  angles  to  the  other 
one  soon  appears  (Fig.  47,  c),  thus  forming  tetrads,  each 
composed  of  four  chromosomes.  The  tetrads  next  shorten, 


108  PHYSICAL  BASIS  OF  HEEEDITY 

opening  out  in  various  ways  to  produce  figures  like  those 
shown  in  Fig.  47,  c. 

The  sex-chromosome  {X)  that  has  no  mate  in  the 
PJirynotettix  male,  and  hence  has  not  conjugated,  has  only 
one  longitudinal  split  (a  dyad).  The  cell,  the  primary 
spermatocyte,  with  its  nucleus  next  divides.  Eleven  auto- 
somes go  to  each  pole,  and  the  sex-chromosome  failing 
to  divide  at  this  time  goes  to  one  daughter  cell  only.  The 
secondary  spermatocytes  are  produced — half  with  12,  half 
with  11  double  chromosomes.  A  short  resting  stage  follows 
— the  chromosomes  again  becoming  diffuse,  i.e.,  forming 
vesicles.  They  soon  reappear  and  a  second  division  takes 
place,  producing  the  spermatids — the  daughter  cells  of 
the  secondary  spermatocytes.  Half  of  these  have  12,  half 
11  chromosomes — the  X-chromosome  having  divided  at 
the  second  division. 

Wenrich  found  it  possible  to  identify  certain  of  the 
chromosomes  and  was  thus  enabled  to  follow  a  few  of 
them  through  several  successive  stages.  Eight  consecu- 
tive stages  in  the  history  of  chromosome  "B^^  of  Phryno- 
tettix  are  shown  in  Fig.  48.  Indications  of  the  primary 
split  are  present  in  a,  b,  c,  the  secondary  split  appears 
first  in  d.  The  evolution  of  the  thread  continues  as  the 
tetrad  becomes  placed  in  the  spindle  in  such  a  way  that 
the  first  separation  of  the  chromosomes  takes  place  along 
the  secondary  split,  i.e.,  the  first  division  is  equational. 
Wenrich  found  in  several  other  individuals  of  this  species 
that  this  same  chromosome  pair  ^'B^'  consist  of  unequal 
members  as  shown  in  Figures  48,  2  a^-li  and  3  a-d.  In 
48,  2  c  a  distinct  crossing  of  the  threads  is  present.  The 
shape  of  the  contracted  chromosome  (/  g  h)  and  its  posi- 
tion on  the  spindle  show  that  one  of  the  longer,  and  one 
of  the  shorter  strands  passes  to  one  pole,  and  similarly  a 
longer  and  shorter  to  the  other  pole.  The  division  here  is 
in  the  plane  of  the  secondary  split,  i.e.,  equational.  The 
inequality  in  length  of  the  conjugating  pair  makes  this 
conclusion  certain  in  this  case. 


CKOSSING  OVER  AND  CHROMOSOMES     109 

In  the  second  division  of  this  chromosome  the  longer 
thread  separates  from  the  shorter  one — the  second  is 
therefore  reductional.    It  is  evident,  especially  from  this 


d 


e 


f 


£ 


h 


d 


e 


f 


g 


h 


a 


d 


a 


Fig.  48. — A  pair  of  chromosomes  "B"  in  conjugation,  1;  the  same  pair  in  conjuga- 
tion in  another  individual  in  which  one  chromosome  is  shorter  than  the  other,  2;  same 
in  a  third  individual,  3;    later  stage  showing  chiasma  of  threads,  4.      (After  Wenrich.) 

last  example,  that  the  crossing  of  the  threads  is  not  an 
indication  that  the  division  of  the  chromosome  is  neces- 
sarily different  from  what  it  is  when  there  is  no  sucli 
crossing.     What  is  more  important  is  that  the  crossed 


110  PHYSICAL  BASIS  OF  HEEEDITY 

threads  furnish  no  proof  that  an  interchange  must  have 
taken  place  earlier,  but  neither  does  it  furnish  any  evi- 
dence that  interchange  had  not  taken  place.  For  example, 
the  most  obvious  interpretation  of  Fig.  48,  2  d  is  that 
the  upper  end  of  the  tetrad  has  separated  in  the  plane 
of  the  secondarj^  split  (in  anticipation,  as  it  were,  of  the 
separation  about  to  take  place  in  this  plane) ,  and  has  sepa- 
rated in  the  lower  part  of  the  same  tetrad  in  the  plane  of 
the  primary  split.  This  interpretation  does  not  involve 
any  real  crossing  over  in  the  sense  that  the  two  crossed 
threads  had  previously  broken  and  interchanged,  as  Jans- 


Fia.  49. — The  same  chromosome  pair  in  conjugation  from  thirteen  different  cells.     (After 

Wenrich.) 

sens'  chiasmatype  assumes  on  the  ground  that  the  two 
granules  (threads)  in  contact  at  the  upper  end  of  the 
tetrad  must  be  related  to  each  other  in  the  same  way  as 
are  those  further  back  in  the  tetrad. 

This  last  assumption  is  the  foundation  of  Janssens' 
view,  but  has  no  longer  sufficient  evidence  to  support  it, 
even  though  none  opposes  it.  Nevertheless,  it  should  be 
clearly  understood  that  evidence  such  as  this,  derived 
from  Wenrich 's  results,  can  not  possibly  be  held  to  show 
that  an  earlier  interchange  or  crossing  over  has  not 
occurred.  If  it  had,  such  a  figure  as  this  (c)  would,  as 
explained  above,  be  a  consequence  to  be  expected. 

The  constancy  of  the  beading  of  the  chromosomes  in 
each  individual  is  most  remarkable.    Its  significance  for 


CROSSING  OVER  AND  CHROMOSOMES     111 

the  linear  order  of  the  material  of  the  chromosomes  cannot 
be  overestimated.  As  a  further  example  Wenrich  gives 
identical  stages  of  the  same  chromosomes  (Fig.  49)  each 
of  the  figures  is  from  a  different  individual.  The  identity 
in  size  and  in  location  of  the  principal  beads  in  the  series 
is  obvious. 

Robertson  has  also  brought  forward  a  case  of  an 
unequal  pair  of  chromosomes  and  interpreted  the  facts  as 
opposed  to  the  crossing-over  hypothesis.  He  found  two 
cases  in  a  grasshopper  of  the  genus  Tettigidea  in  which 
there  was  a  very  unequal  pair  of  chromosomes.  The 
shorter  piece  conjugated  consistently  with  only  one  part 
of  the  longer  chromosome,  as  shown  in  the  next  figure 


a 


fmmm. 


d 


Fig.  50 — Conjugation  of  an  unequal  pair  of  chromosomes  and  their  subsequent  separation. 

(After  Robertson.) 

(Fig.  50,  a,h).  At  the  first  maturation  division  the  two 
chromosomes  separated,  as  shown  in  (c,  ^,  e).  It  would  be 
difficult  to  find  a  more  excellent  illustration  of  the  per- 
sistence of  the  individuality  of  the  chromosomes  after  con- 
jugation, and  the  case  falls  equally  in  line  with  the  view 
that  conjugation  takes  place  only  between  those  parts  of 
the  chromosome  that  are  alike,  i.e.,  composed  of  the  same 
series  of  genes.  How,  then,  could  this  case,  so  admirably 
suited  to  support  the  chromosome  theory  be  turned  against 
the  chiasma  theory  %  Only,  I  think,  through  a  misconception 
of  the  essence  of  the  theory.  Robertson  says:  *^In  both 
types  of  unequal  tetrads  we  have  very  strong  evidence  that 
the  homologous  chromosomes,  on  entering  the  side-to-side 
pairing  process  of  synapsis,  remain  as  distinct  individ- 
uals, retain  their  identity  throughout  the  period,  and  come 


112        PHYSICAL  BASIS  OF  HEREDITY 

out  of  it  with  at  least  the  same  size  they  had  on  entermg 
it.  Each  pairing  chromosome  maintains  its  distinct  indi- 
viduality during  this  period.  This  is  opposed  to  the 
idea  of  Janssens  ( '09)  and  Morgan  ( '11),  as  expressed  in 
the  theory  of  chiasmatype.  In  their  theory  they  assume 
that  homologous  chromosomes  in  parasynapsis  twist  about 
each  other  and  fuse.  On  splitting,  a  plane  passes  down 
the  fused  body,  regardless  of  the  previous  spiral  fusion 
plane,  resulting  in  two  daughter  chromosomes  which  may 
not  be  identical  with  the  two  chromosomes  which  entered 
the  process.  Each  new  one  may  contain  parts  of  both 
original  chromosomes.  If  such  had  been  the  case,  the 
separation  or  formation  of  a  short  and  a  long  chromo- 
some out  of  the  first  chromosome  with  such  regular- 
ity of  size,  etc.,  as  we  have  shown,  could  not  have 
occurred."  On  the  contrary,  even  if  crossing  over  had 
occurred  within  the  region  where  the  short  and  the 
long  pieces  came  together,  the  separation  would  be 
expected  still  to  be  exactly  that  described  by  Robertson; 
for  the  genetic  evidence  points  very  clearly  to  the  con- 
clusion that  the  interchange  involves  exactly  equal  and 
opposite  parts.  There  is  no  reason  to  suppose  that 
regions  outside  the  conjugating  region  would  be  affected ; 
on  the  contrary,  all  the  genetic  evidence  would  lead  us  to 
expect  no  such  effects. 

Summary  of  Evidence 

If  we  have  found  Janssens'  evidence  inadequate  as  a 
demonstration  of  crossing  over,  what  other  evidence  is 
there  in  the  history  of  the  chromosome  to  which  an  appeal 
can  be  made?  First,  there  is  the  undisputed  fact  that  at 
the  time  when  the  chromosomes  come  together  they  spin 
out  into  long,  thin  threads  which,  as  they  meet,  lie  over 
and  under  each  other,  so  that  the  line  of  fusion  is  in  a 
spiral  plan.  Later,  when  the  fusion  is  complete,  it  is  no 
longer  possible  to  follow  the  plane  of  union,  but  unless 
the  chromosomes  slip  around  each  other  after  crossing 


CROSSING  OVEE  AND  CHROMOSOMES     113 

over — for  which  there  is  no  evidence — one  member  of  tlie 
pair  must  lie  on  one  side  of  its  mate  in  one  region,  and 
on  the  other  side  in  other  regions.  Second,  when  the  thick 
thread  splits  anew  just  before  condensing  into  the  tetrad 
it  is  so  difficult  to  follow  the  course  of  the  split  in  all  cases 
that  it  cannot  be  affirmed  that  it  always  lies  in  one  plane 
throughout  the  length  of  the  chromosome,  but  if  such 
should  turn  out  to  be  the  case,  as  so  often  figured,  it  would 
appear  to  mean  that  the  crossing  over  had  taken  place  and 
been  obliterated  by  the  time  the  condensation  began. 
Third,  evidence  such  as  that  described  by  Wenrich — of 
which  sort  there  are  other  cases  but  none  quite  so  clear — 
indicates  that  the  chromosomes  are  enclosed  in  vesicles 
until  they  begin  to  spin  out  each  into  a  long  thread.  Inter- 
change of  the  sort  called  for  by  the  genetic  evidence  could 
scarcely  take  place  until  the  walls  of  the  sacs  had  disap- 
peared. The  thin  thread  stage  that  follows  would  seem 
best  to  fulfill  the  conditions  called  for  by  the  genetic  evi- 
dence. The  moment  the  primary  split  appears  after  the 
two  threads  have  fused  there  would  seem  to  be  precluded 
any  further  chance  for  crossing  over,  as  the  genetic 
evidence  suggests.  This  analysis  leads,  then,  to  the  thin- 
thread  stage  as  the  most  favorable  stage  for  the  requir- 
ments  of  the  genetic  evidence. 

It  is  well  known  that  most  of  our  information  about  the 
maturation  stages  is  derived  from  the  male,  because  of  the 
greater  ease  of  obtaining  the  critical  stages,  and  in  prepar- 
ing material.  We  are  handicapped  in  discussing  crossing 
over  to  a  large  extent  by  the  fact  that  we  must  appeal 
largely  to  the  evidence  of  spermatogenesis.  In  DrosopJiila 
at  least  there  is  no  crossing  over  in  the  male.  On  the 
other  hand,  Nabours  has  recently  found  evidence  in  one  of 
the  grasshoppers  that  crossing  over  occurs  both  in  the 
male  and  female.  In  this  case  evidence  from  the  male 
would  be  more  to  the  point.  Whether  genetic  crossing 
over  occurs  in  the  male  of  Batracoseps  and  Tomopteris, 
we  do  not  know. 

8 


114  PHYSICAL  BASIS  OF  HEEEDITY 

In  the  female  of  some  insects,  amphibians,  selach- 
ians and  annelids,  the  thin-thread  stages  in  the  form  of 
U-shaped  loops  have  been  described — stages  that  are  so 
much  like  those  of  the  male  that  the  argument  for  one 
would  seem  to  extend  to  the  other.  Cut  again  this  proves 
too  much,  and  we  have  yet  to  learn  what  cytological  dif- 
ferences exist  in  cases  where  crossing  over  occurs  in  one 
sex  and  not  in  the  other.  On  the  whole,  then,  while  the 
genetic  evidence  is  favorable  in  all  essentials  to  the  theory 
of  interchange  between  homologous  chromosomes,  it  must 
be  confessed  that  the  cytological  evidence  is  so  far  behind 
the  genetic  evidence  that  it  is  not  yet  possible  to  make  a 
direct  appeal  to  the  specific  mechanism  of  crossing  over  on 
the  basis  of  our  cytological  knowledge  of  the  maturation 
stages.  The  idea  that  the  chromosomes  disappear  as  such 
and  go  into  some  sort  of  suspension  during  the  resting 
stage  is  an  old  idea.  0.  Hertwig  thought  that  the  chromo- 
somes did  actually ' '  dissolve ' '  at  this  time  and ' '  recrystal- 
lize"  at  each  division  stage.  Goldschmidt  elaborated  a 
view  of  crossing  over  that  rests  on  the  assumption  that 
the  homologous  genes  are  set  free  in  the  resting  nucleus 
and  may  become  interchanged  during  reconstruction. 
Aside  from  certain  inherent  contradictions  in  Gold- 
schmidt's  scheme  (the  most  obvious  ones  have  been 
pointed  out  by  Sturtevant  and  by  Bridges),  it  stands 
in  contradiction  to  the  one  most  certain  fact  that  we 
know  about  crossing  over,  viz.,  that  not  single  genes 
but  whole  blocks  of  genes  are  involved — in  fact,  the  most 
common  sort  of  interchange  involves  the  tw^o  entire  pieces 
of  each  chromosome. 

The  general  idea  that  the  genes  become  dissociated 
during  the  resting  phases  is  disproven  by  the  wsij  in 
.  which  they  come  together.  The  genetic  evidence  from 
-f^Drosophila  shows  that  when  crossing  over  occurs,  let  us 
say  at  the  middle  of  the  chromosome,  all  of  the  genes  of 
each  half  of  each  pair  hold  together — and  exchange  as 
large  pieces.     Now  if  the  genes  are  dissolved  at  each  rest- 


CROSSING  OVEE  AND  CHROMOSOMES     115 

ing  stage,  there  can  be  given  no  explanation  as  to  \vhy 
homologous  genes  should  not  recombine  in  all  possible 
combinations  with  other  genes.  But  this  is  exactly  what 
does  not  happen.  If  it  be  supposed  that  the  chromosomes 
dissolve  only  partly  into  chains  of  genes,  it  is  still  not 
obvious  why  the  chains  of  one  chromosome  should  be  iden- 
tical with  those  of  the  other  (its  homologue)  as  they  must 
be  to  recombine  properly;  for,  in  neighboring  nuclei  other 
chains  are  forming — as  the  crossing-over  results  indi- 
cate— involving  breaking  at  all  possible  levels. 

Bateson  and  Punnett  have  proposed  a  theory  of  cross- 
ing over  that  is  called  reduplication.  It  is  fundamentally 
different  from  the  one  here  adopted.  Although  I  think 
this  theory  outlawed  by  the  evidence  that  Plough  has 
obtained,  and  made  impossible  by  certain  other  considera- 
tions that  will  be  given  later,  the  theory  is  so  interesting 
that  it  may  be  briefly  stated.  Bateson  suggests  that  at 
some  time  early  in  the  embryo  segregation  may  take  place 
involving  heterozygous  pairs  of  factors.  In  the  actual 
case  presented  only  two  such  pairs  are  involved.  As  a 
'^symbolic  presentation '^  of  the  situation  Bateson  gives 
the  diagram  drawn  in  Fig.  51. 

Although  the  dichotomous  method  of  separation  is 
utilized  in  the  second  line  of  figures  to  show  reduction  of 
the  two  pairs  at  once,  such  figures  could  obviously  bear 
no  relation  to  the  ordinary  process  of  cell  division — nor 
do  they,  I  understand,  pretend  to  be.  After  separa- 
tion (segregation)  the  cells  that  get  AB  and  ah  are  repre- 
sented as  dividing  faster  than  the  cells  Ah  and  Ba,  hence 
there  will  be  more  of  them  in  proportion  as  the  two  rates 
of  division  differ. 

Bateson 's  view  is  open  to  the  following  criticisms: 

1.  The  evidence  from  Drosophila,  where  many  linkage 
ratios  are  known,  gives  no  support  to  the  view  that  these 
ratios  fall  into  relatively  few  dichotomous  schemes,  such 
as  Bateson 's  hypothesis  calls  for.  Other  forms  also  fail  to 
fit  such  a  view.    On  the  contrary,  the  ratios  fall  into  no 


116 


PHYSICAL  BASIS  OF  HEREDITY 


such  groups  as  those  given  by  Bateson.  Even  were  it  pos- 
sible to  suppose  that  in  each  case  a  different  reduplica- 
tion occurred  (Le.,  a  different  number  of  generations  was 
passed  through),  still,  as  said  above,  it  is  not  obvious 
that  the  linkage  series  stands  in  any  such  numerical  {i.e., 
dichotomous)  relation  as  the  view  demands. 


ABxab 
(       AB  ab       ) 


i^bxaB 


(      Ab.aB      ) 


n-i 


n-1 


3AB 


IBa 


lAB 


Fig.  51. — Two  schemes  illustrating  the  idea  of  reduplication  by  Bateson  and 
Punnett;  the  three  figures  to  the  left  illustrating  "coupling,"  and  the  three  to  the  right 
"repulsion." 

2.  If  reduplication  occurred  at  an  early  stage  in  the 
germ  tract,  we  should  expect  to  find  in  any  organ  of  limited 
size,  as  a  stamen,  that  there  would  be  a  likelihood  that  it 
would  contain  for  the  most  part  a  particular  kind  of  cell. 
Altenburg  tested  out  this  view  with  pollen  of  the  primrose 
and  found  no  evidence  in  favor  of  a  limited  distribution — 
on  the  contrary,  he  found  that  all  the  linkage  combinations 


CROSSING  OVER  AND  CHROMOSOMES     117 

were  present  in  each  stamen  in  the  expected  proportions. 
These  and  other  difficulties  make  it  improbable  that  link- 
age can  be  the  result  of  this  kind  of  reduplication. 

Bateson  and  Punnett  formulated  their  hypothesis  at 
first  for  only  two  pairs  of  linked  factors.  When  it  was 
shown  that  three  pairs  of  factors  could  show  linkage, 
Bateson  and  Punnett  assumed  that  all  three  pairs  of  fac- 
tors might  segregate  at  the  same  time  (or  in  three  suc- 
cessive divisions),  the  observed  ratios  being  due,  as 
before,  to  unequal  division  rates  later.  Trow  has  sug- 
gested that  in  such  cases  the  segregation  and  reduplica- 
tion for  the  third  pair  of  factors  might  not  occur  until 
that  for  the  first  two  pairs  was  completed.  This  view 
seemed  to  meet  certain  inadequacies  of  the  former  hypoth- 
esis, but  meets  with  certain  difficulties  on  its  own  account. 
One  of  the  most  obvious  of  these  objections  is,  as  Sturte- 
vant  has  pointed  out,  that  the  number  of  cell  divisions, 
necessary  to  produce  some  of  the  higher  ratios  that  are 
known,  would  produce  a  mass  of  cells  thousands  of  times 
larger  than  the  animal  itself. 


CHAPTER  IX 
THE  OEDER  OF  THE  GENES 

The  proof  of  the  linear  order  of  the  genes  is  derived 
di]-ectly  from  the  linkage  data.  It  is  not  dependent  on  the 
chromosome  theory  of  heredity.  Fortunately,  as  was 
]^ointed  out  in  the  last  chai)ter,  there  are  many  facts  about 
the  maturation  stages  of  the  eggs  and  sperm  that  fit  in 
extraordinarily  well  with  the  theory  of  the  linear  order 
of  the  genes,  but  let  me  repeat,  the  proof  of  the  order  is 
not  dependent  on  the  chromosomal  situation.  The  evi- 
dence for  the  linear  order  is  furnished  by  linkage  and  its 
correlative  phenomenon,  crossing  over.  By  linkage  is 
meant  that  certain  factors  that  enter  the  cross  from  each 
parent  remain  together  in  subsequent  generations,  more 
often  than  they  separate.  For  example,  if  in  Drosophila 
yellow  wings  and  white  eyes  have  entered  from  one  parent 
and  gray  wings  and  red  eyes  from  the  other,  the  new 
(crossover)  combinations,  yellow  and  red,  gray  and 
white,  are  less  numerous  than  are  the  original  linked 
combinations,  yellow  and  white,  gray  and  red.  The  num- 
ber of  individuals  (crossovers)  that  result  from  this 
interchange,  expressed  as  percentage  of  the  whole  number 
of  indi\dduals,  is  called  tlie  crossover  value.  Such  a 
percentage  indicates  how  often  the  linkage  is  broken. 
Thus,  if  crossing  over  between  yellow  and  white  is  shown 
in  1  per  cent,  of  the  gametes,  then  1  stands  for  the  cross- 
over value  of  yellow  and  white.  Conversely,  yellow  and 
white  have  remained  together  (linked)  in  99  per  cent,  of 
the  gametes.  We  speak  of  the  linkage  relations  in  such 
cases  in  terms  of  the  crossover  values,  here  1  per  cent. 

For  the  proof  of  the  linear  order  of  the  genes,  it  is  only 

118 


THE  ORDER  OF  THE  GENES  119 

necessary  to  represent  one  set  of  linked  genes  (a,  b,  c,  etc), 
ignoring  the  normal  allelomorphic  series,  for  these  follow 
the  same  (reciprocal)  changes. 

If  a,  h,  and  c  stand  for  three  genes,  and  if  the  linkage 
relations  of  a  to  h  and  of  &  to  c  are  known,  the  relation 
of  a  to  c  is  a  function  of  the  sum  of  ah  and  he  or  of  the 
difference  of  ah  and  he.  For  example,  if  the  crossover 
value  ah  is  expressed  as  5,  and  that  of  he  as  10,  then  ac 
is  a  function  of  the  sum  (15),  or  the  difference  (5)  of  ah 
and  he.  It  cannot  be  said  that  ae  must  be  5  or  15  because 
another  possible  process  may  intervene  to  affect  the  sum 
or  the  difference,  viz.,  double  crossing  over  in  the  region 
involved.  By  making  the  distance  so  small  that  double 
crossing  over  is  practically  excluded  the  sum  or  the  dif- 
ference is  actually  the  realized  result,  as  the  following 
example  illustrates : 

When  three  mutant  characters  yellow,  white  and 
bifid  were  all  used  together  in  a  single  experiment,  it 
was  found  that  there  w^ere  1160  non-crossovers,  15  flies 
representing  single  crossovers  between  yellow  and  white, 
and  43  flies  representing  single  crossovers  between  white 
and  bifid.  There  were  no  flies  representing  crossing  over 
in  both  regions  at  the  same  time,  i.e.,  there  were  no  double 
crossovers.  Thus  the  crossover  value  yellow  white  is 
1.2,  and  the  crossover  value  w^hite  bifid  is  3.5.  The  same 
data  give  the  yellow  bifid  crossover  value  of  4.7,  which  is 
precisely  the  sum  of  the  two  component  values : 


yellow 


The  simplest  way  in  which  such  a  relation  can  be 
thought  of  is  that  the  three  genes  stand  in  a  line.  Suppose 
a  fourth  linked  gene,  d,  is  added  to  the  series.  It  is  then 
found  that  hd,  is  a  function  of  the  sum  or  of  the  differ- 


120  PHYSICAL  BASIS  OF  HEEEDITY 

ence  of  &  to  c  and  c  to  d.  Four  points  arranged  in  a 
straight  line  still  fulfill  the  relations  here  found.  I  know 
of  no  other  geometric  configuration  that  covers  all  these 
results — perhaps  there  is  none.  When  we  add  more 
and  more  linked  genes  to  the  series,  and  find  the  same 
predictable  relations  continue  to  hold,  the  theory  of 
the  linear  arrangement  becomes  firmly  established.  Per- 
haps the  best  proof  of  the  linear  order  is  found  in  the 
opportunity  it  gives  for  prediction;  for,  when  the  rela- 
tion oi  d  to  h  and  to  c  is  known  its  relation  to  a  can  be 
foretold  accurately. 

It  has  been  found  when  there  is  a  large  amount  of  cross- 
ing over  between  two  factors  used  in  an  experiment,  that 
the  crossover  value  is  not  the  same  as  the  value  deter- 
mined by  adding  together  the  crossover  values  of  inter- 
mediate points  between  the  two  factors  in  question.  What 
appears  here  to  be  a  contradiction  proves,  when  under- 
stood, to  be  one  of  the  best  pieces  of  evidence  in  support 
of  the  theory  of  the  linear  order. 

A  few  examples  will  serve  to  illustrate  the  point  at 
issue.  When  a  fly  with  yellow  wings  and  bar  eyes  is  mated 
to  a  wild-type  fly,  the  amount  of  crossing  over  in  the  F^ 
female  between  yellow  and  bar  (as  determined  by  back- 
crossing)  is  43.6  per  cent.,  but  if  the  crossing  over  between 
yellow  and  bar  is  calculated  by  adding  up  the  crossover 
values  obtained  by  using  intermediate  points  {ah  +  he, 
etc.)  the  value  is  about  57  per  cent.  The  apparent  incon- 
sistency is  at  once  cleared  up  by  arranging  the  experiment 
so  that,  while  obtaining  the  data  for  yellow  and  bar, 
there  are  also  obtained  data  showing  what  is  happening  in 
the  region  between  them.  It  is  found  that  a  large  amountof 
double  crossing  over  occurs,  and,  when  the  correction  for 
this  is  made,  the  "discrepancy''  disappears.  If  crossing 
over  may  take  place  at  any  level,  it  is  obvious  that  it  might 
occur  at  two  points  at  the  same  time,  and  experience 


THE  OEDER  OF  THE  GENES 


121 


shows  that  such  is  the  case,  for  such  double  crossing  over 
can  be  detected  if  enough  points  in  the  series  are  '' in- 
volved'^  to  catch  all  single  crossovers.  Now,  as  shown 
in  Fig.  52,  whenever  double  crossing  over  takes  place 
between  y  and  B,  the  two  series  that  result,  as  marked 
by  their  ends  alone  {y  and  B),  are  still  y  and  B.  The  flies 
will  therefore  be  placed  in  such  classes,  which  are  the  non- 
crossover  classes.  A  numerical  increase  in  this  class 
will  decrease  the  calculated  percentage  of  crossovers. 
Thus  double  crossing  over  by  increasing  the  number  of 


y 


B 


B 


Fig.   52. — Scheme  illustrating  how  double  crossing  over  between  two  distinct  genes,  y  and 
B,  is  not  recorded,  when  only  y  and  B  are  involved. 

apparent  non-crossovers,  decreases  the  observed  per- 
centage of  crossovers.  When  enough  points  are  marked 
along  the  series  to  pick  up  all  double  crossovers,  and 
these  are  then  referred  to  the  proper  single  crossover 
classes,  the  ^  ^  piece-by-piece ' '  per  cent,  estimate,  and 
the  percentage  obtained  from  the  cross,  are  found  in 
complete  agreement. 

The  amount  of  double  crossing  over  in  Drosojjhila  is  so 
large  that  the  percentage  of  '' crossing  over"  is  rarely  or 


122  PHYSICAL  BASIS  OF  HEREDITY 

never  more  tlian  50  per  cent.,  altliough  the  actual  num- 
bers given  for  ^'distances''  between  two  genes  may  be  as 
much  as  107  based  on  summation  of  short  distances.  The 
latter  method  of  calculation  is  the  accurate  way  of  stating 
the  result,  and  whenever  possible  it  is  adhered  to,  i.e., 
the  percentage  numbers  for  crossing  over  are  sum  totals 
based  on  results  obtained  with  genes  so  near  together  that 
double  crossing  over  is  practically  excluded. 

Another  illustration  where  the  ditference  between  the 
direct  calculation  between  two  factors  (scute  and  forked) 
and  the  '^  piece-by-piece  "  estimate  is  greater  than  50,  is  as 
follows :  At  one  end  of  the  series  of  sex-linked  genes  is  a 
factor  scute  (zero)  and  near  the  other  end  forked.  The 
direct  data  for  crossing  over  between  them  gave  a  cross- 
over value  of  48.2.  Between  them  three  other  loci  were 
present  in  the  same  experiment,  and  crossing  over  between 
them  could  also  be  detected.  As  shown  in  the  table  below, 
the  sum  of  these  crossover  values  gave  61.1  units  between 
scute  and  forked. 

Scute 


48.2 
Garnet 


14.2 

Forked 


The  presence  of  the  intermediate  factors  makes  it  pos- 
sible to  pick  up  most  of  the  double  crossing  over  that 
occurred  between  scute  and  forked.  When  a  correcticm 
is  made  for  these  the  diiference  between  48.2  and  61.1 
entirely  disappears.  Another  and  still  more  extreme 
example  will  help  to  make  this  more  evident.  Near  one 
end  of  the  second  chromosome  is  the  gene  for  star  (eyes), 


THE  ORDER  OF  THE  GENES 


12:] 


near  tlie  other  end  is  the  gene  for  speck.  Bridges  fur- 
nishes the  following  data  in  regard  to  crossing  over 
between  these  loci.  When  only  these  distant  loci  are 
used  the  crossover  value  is  48.7.  When  the  sum  of  the 
crossover  values  between  the  following  seven  genes  is 
taken  as  the  value  for  star  and  speck  it  amounts  to  1U4.4. 


104.4 


star 


28.3 


Dachs 


Black 


Purple 
Vestigial 


Curved 


30.2 


Speck        y 


48.7 


In  this  case  (as  other  experiments  show)  there  are  still 
two  units  missing  in  the  map  distance  given  above  (104.4), 
because  of  one  per  cent,  of  double  crossovers  in  the 
region  between  curved  and  speck,  that  are  not  here 
recorded,  since  there  are  here  no  loci  within  this  distance 
of  30.2. 

Whenever  cases  in  which  double  crossing  over  ha^ 
taken  place  are  checked  up  as  in  the  foregoing  cases,  it  is 
found  that  the  discrepancies  in  the  two  methods  are 
accounted  for. 

It  is  instructive  to  compare  the  preceding  case  with 
another  one  including  several  of  the  same  genes,  but  in 
addition  something  else  (deficiency),  that  cuts  down  the 
amount  of  crossing  over  in  certain  regions.  The  cross- 
over value  between  star  and  speck  was  found  to  be  47.7 
in  this  experiment.  The  sum  of  the  crossover  values  of 
the  six  loci  involved  gave  79.3  units.     The  difference 


124 


PHYSICAL  BASIS  OF  HEREDITY 


between  47.7  and  79.3  is  due  to  double  crossovers  as  the 
data  for  the  intermediate  regions  show: 


star 


24.1 


79.3 


Dachs  deficiency 


Black 


47.7 


Purple 


Vestigial 


Speck 

There  was  known  to  be  present  in  this  case  a  factor  called 
^ '  deficiency '  ^  in  one  of  the  two  second  chromosomes 
involved.  It  is  near  ' '  dachs ' '  and  cuts  down  the  crossing 
over  between  star  and  speck  by  about  25.8  units.  It 
will  be  noticed  that  while  the  second  summation  value 
for  star  speck,  viz.,  79.3,  and  the  first  value,  viz.,  104.4,  are 
very  different,  the  crossover  value  between  star  and  speck 
is  in  one  case  47.7  and  in  the  other  48.7.  The  meaning  of 
this,  as  shown  by  data  for  intermediate  loci,  is  that  by  the 
addition  of  25.1  units  (104.4  minus  79.3  equals  25.1)  the 
number  of  double  crossovers  has  so  greatly  increased  that 
a  difference  of  only  about  1  per  cent,  of  apparent  crossing 
over  is  recorded  in  the  star  speck  value. 


"Distance"  and  Linear  Order 

The  linear  order  of  the  genes  implies  distance  between 
them,  for  w^hich  the  crossover  values  stand  as  indices.  It 
is  obvious  that  if  the  order  of  the  genes  remained  the  same 
but  something  doubled  the  number  of  crossovers  between 
two  loci,  their  ''distance'^  apart  would  at  the  same  time 
appear  to  have  been  doubled.  Again,  if  crossing  over  is 
thought  of  as  due  to  twisting  of  the  chromosomes  of  a  pair 
about  each  other,  then  if  the  twisting  is  more  likely  to  occur 
at  the  ends  of  the  chromosomes,  or  if  the  twists  themselves 


THE  OEDER  OF  THE  GENES  125 

are  shorter  there,  '^ distance"  in  these  regions  is  on  a  dif- 
ferent scale  from  distance  in  the  middle  of  the  same 
chromosome.  Factors  for  crossing  over  have  been  found, 
by  Sturtevant,  that  change  the  values  in  certain  parts  of 
the  series  and  leave  other  parts  unaffected.  When  the 
influence  of  these  special  genes,  that  can  be  treated  in  the 
same  way  as  are  all  Mendelian  genes,  is  removed,  the 
region  that  was  affected  gives  its  original  crossover 
values  again. 

It  is  to  be  understood,  then,  that  when  we  substitute 
the  idea  of  distance  for  crossing  over  values  the  term  is 
not  used  in  an  absolute  sense,  but  in  a  relative  sense,  and 
that  it  depends  always  on  the  conditions  of  the  experiment. 
That  the  genes  do  stand  at  definite  levels  in  the  chromo- 
somes, and  that  in  this  sense  they  are  definitely  spaced, 
seems  reasonable  in  the  light  of  all  the  evidence  bearing 
on  this  point;  but  even  if  they  are  so  spaced  that  crossing 
over  is  a  function  of  their  distance  from  each  other  in  the 
series,  any  influence  that  determines  how  often  inter- 
change between  homologous  pairs  will  take  place  would 
give  the  appearance  that  the  actual  distances  themselves 
have  changed. 


CHAPTER   X 
INTERFERENCE 

One  of  the  most  significant  results  that  a  study  of 
crossing  over  has  brought  to  light  is  that  whole  blocks 
of  genes  go  over  together.  Thus,  if  one  series  he  A  B  C  D 
E  F  G  H I  J  K  L  M  N  and  its  allelomorphic  series  be  a  b  c 
d  e  f  g  h  i  j  hi  m  n  crossing  over  may  give  two  blocks  of 
genes. 

A  B  C  D  E  f  0   h  i  j  Ic    I  m  n 
a  b  c    deFGHIJKLMN 

This  result  can  best  be  demonstrated  in  cases  where  a 
number  of  loci  are  followed  at  once. 

The  fact  that  crossing  over  takes  place  in  blocks  is 
highly  significant  for  the  phenomenon  of  distribution, 
since  it  means  that  pairs  of  linked  genes  do  not  act  inde- 
pendently of  their  neighbors.  This  fundamental  relation 
was  not  suspected  until  quite  recently. 

The  size  of  the  blocks,  when  only  one  crossing  over 
occurs  between  the  chromosome  pairs,  depends  on  the  loca- 
tion in  the  series  of  the  breaking  point.  If  the  crossing 
over  occurs  near  the  middle,  the  four  pieces  will  be  of  the 
same  length  as  shown  below : 

abcdefgHIJKLMN 
ABCDEFGhijhlmn 

If  it  is  near  the  end  of  the  series,  two  of  the  resulting 
pieces  will  be  small,  the  other  two  large.    Thus : 

a b c DEFGHIJKLMN 

A  B  C    d    e   f    rj    h   i   j   k   I    m>   n 

The  two  ''like''  pieces  in  all  cases  contain  identical  series 
of  loci. 

126 


INTERFERENCE  127 

The  data  also  show  that  the  series  may  break  at  two 
points,  and  that  when  this  happens  the  three  blocks  oi'  one 
set  always  correspond  to  the  three  blocks  of  the  other 
series  of  genes.     Thus  interchange  at  two  levels  gives : 

ahcdEFGHijkl 
ABCDefghlJKL 

The  same  relation  holds  in  principle  for  three  or  more 
breaks  in  the  series. 

If  in  such  a  system  the  blocks  have  no  commonest 
length,  the  break  in  the  series  at  one  level  should  not 
bear  any  relation  to  the  place  at  which  another  break 
takes  place.  For  example,  if  it  is  true  that  when  a  break 
occurred  between  D  and  E  it  had  no  influence  on  a  break  at 
any  other  point  of  the  series,  the  blocks  resulting  from  two 
breaks  would  not  tend  to  be  more  of  one  length  than  of  any 
other  length.  But  if  the  evidence  shows  that  when  a 
break  occurs  between  D  and  E  the  chance  of  another  break 
occurring  in  that  vicinity  is  decreased,  or  increased,  the 
results  vvould  be  expected  to  follow  some  definite  law  or 
principle,  rather  than  be  simply  the  result  of  chance.  This 
IS  in  fact  the  case.    An  illustration  may  make  this  clear. 

Suppose  when  crossing  over  takes  place  within  the 
blocks  A  B  C  D,  and  E  F  G  H,  and  I  J  K  L  it  can  be 
recorded.  If  we  know  how  often,  when  the  break  occurs 
only  once  in  the  series,  it  takes  place  in  the  first,  in  the 
second,  or  in  the  third  block,  we  can  then  determine  in 
those  cases  where  breaking  occurs  in  the  first  block, 
whether  it  is  as  likely  to  take  place  in  the  second  block  as 
when  no  break  occurs  in  the  first,  etc.  Such  tests  have 
been  made  (Muller,  Sturtevant,  Bridges,  Weinstein, 
Gowen)  with  Drosophila,  and  the  same  kind  of  results  con- 
sistently obtained.  It  has  been  found,  for  example,  that 
when  a  crossing  over  takes  place  between  G  and  H,  a  sec- 
ond one  is  less  likely  to  take  place  on  either  side,  i.e., 
between  F  and  G  or  between  H  and  /  than  when  no  cross- 


128  PHYSICAL  BASIS  OF  HEREDITY 

ing  over  takes  place  between  G  and  H.  Stated  in  another 
way,  crossing  over  in  one  region  protects  neighboring 
regions  from  crossing  over.  Moreover,  this  relation  fol- 
lows a  perfectly  definite  law  according  to  the  "distances," 
as  determined  by  linkage  relations  of  genes  outside  of  the 
region  of  crossing  over.  If  we  take  two  pairs  of  factors 
closely  linked  together  we  find  that  the  genes  lying 


G  H 

g    h 


immediately  to  the  right  and  left  of  ^  never  cross  over 
independently  of  -  and  ~  at  the  time  that  a  crossover 
separates  -  and  f .  In  other  words,  the  genes  imme- 
diately to  the  right  of  H  always  go  over  with  Hy  and  those 
to  the  left  of  G  always  go  over  with  (7,  when  G  separates 
from  H, 

If  we  consider  genes  that  are  less  closely  linked  with 
G  and  with  Hj  we  find  that  while  their  crossing  over  is 
interfered  with  by  the  crossing  over  between  G-Hj  it  is 
affected  to  a  limited  extent.  G-enes  still  less  linked  with  G 
or  with  H  are  still  less  interfered  with ;  until  finally  there 
is  no  relation  at  all  between  crossing  over  between  G-H, 
and  other  more  loosely  linked  genes,  i.e.,  crossing  over 
between  G-H  is  found  to  have  no  relation  to  crossing  over 
between  L  and  M.  Put  in  another  way,  one  may  say  that 
crossing  over  at  L  and  M  is  no  more  likely  to  take  place 
when  none  occurs  between  G-H,  than  when  it  does. 

For  different  pairs  of  chromosomes  the  regions  that 
bear  this  relation  to  each  other  have  been  found  to  be 
different.  Even  within  the  same  chromosome  this  rela- 
tion may  be  different  at  the  ends  and  in  the  middle.  There 
are  also  special  factors  that  affect  special  chromosomes 
and  special  regions  of  chromosomes.  An  example  will 
illustrate  this  relation  that  is  called  interference.  If  in 
a  group  of  genes  A  B  C  D  E  F  3,  break  occurs  somewhere 
between  A  and  D  in  6  per  cent,  of  cases,  and  if  between 
M  and  T  in  the  same  series  {M  N  0  P  Q  R  S  T),  inlOipeT 
cent,  of  cases,  a  double  break  involving  both  regions  simul- 
taneously should,  if  the  breaks  occurred  independently  of 


INTERFERENCE  129 

each  other,  take  place  in  0.6  per  cent,  of  the  cases.  But 
if  the  regions  in  question  are  close  together,  that  is,  if  the 
intervening  block  (i.e.,  G  F  E  J  K  L)  oi  genes  is  short,  it 
is  found  that  there  are  fewer  double  crossovers  than  the 
0.6  per  cent,  expected  on  a  purely  random  basis.  This 
was  shown  by  Sturtevant  in  his  paper  on  chromosome 
maps.  It  means  that  a  break  in  one  region  interferes 
with  a  break  in  the  other  region  when  the  intervening 
block  is  short. 

The  ratio  of  the  number*  of  actual  double  breaks 
obtained  to  the  number  of  double  breaks  that  would  occur 
if  one  of  them  did  not  interfere  with  the  other  is  termed 
coincidence.  If  in  the  above  example  only  0.3  per  cent, 
of  the  cases  were  double  crossovers  involving  the  regions 
ABCDEF  SiTid  MN0PQR8T  the  coincidence  would 
be  0.3  per  cent,  divided  by  0.6  per  cent.,  or  0.5. 

It  has  been  found  that  as  the  distance  between  two 
regions  increases,  crossing  over  in  one  of  them  interferes 
less  and  less  with  crossing  over  in  the  other;  that  is,  the 
number  of  double  crossovers  obtained  approaches  the 
number  expected  on  a  random  basis,  and  coincidence  rises 
gradually  to  the  value  of  1.  This  phenomenon  is  shown 
in  all  the  cases  where  more  than  one  block  of  genes  has 
been  followed.  It  is  especially  clear  in  the  work  of  Muller, 
who  studied  a  large  number  of  factors  in  the  sex-chromo- 
some of  Drosophila  simultaneously. 

When  the  intervening  block  becomes  sufficiently  long 
so  that  the  coincidence  attains  the  value  of  1,  interference 
has  entirely  disappeared.  When,  however,  the  distance  is 
increased  still  further  interference  reappears,  i.e.,  coin- 
cidence decreases  again.  There  was  a  suggestion  of  this 
in  Muller 's  work;  and  the  work  of  Weinstein  undertaken 
to  get  critical  evidence  on  this  point  indicates  clearly  that 
such  a  decrease  exists.  For  the  second  chromosome  a 
similar  rise  and  fall  with  increase  of  distance  is  indicated 
by  Bridges '  data. 

9 


130  PHYSICAL  BASIS  OF  HEREDITY 

The  fact  that  interference  reappears,  i.e.,  that  coinci- 
dence decreases  after  reaching  a  maximum,  indicates  that 
the  segment  of  a  chromosome  between  the  breaking  points 
tends  to  be  of  a  particular  (modal)  length;  and  that  breaks 
which  are  closer  together  or  farther  apart  than  this  modal 
length  are  less  frequent.  That  is,  genes  not  only  stick 
together  in  blocks,  but  the  blocks  tend  to  be  of  a  definite 
size,  and  longer  and  shorter  blocks  are  less  frequent. 
In  the  sex-chromosome  of  Drosophila,  which  is  65  units 
long,  Weinstein's  data  indicate  that  the  most  frequent 
length  of  block  is  about  46.  In  the  second  chromosome 
(which  is  107  units  long).  Bridges^  data  indicate  a  modal 
length  of  about  15  in  the  centre  of  the  chromosome  and 
of  about  30  on  either  side  of  the  middle  point. 

The  work  on  coincidence  throws  light  on  the  behavior 
of  the  chromosomes  during  crossing  over.  The  cytologi- 
cal  evidence  has  not  determined  whether  when  crossing 
over  takes  place  the  chromosomes  are  twisted  loosely  or 
tightly.  But  Muller  has  shown  that  this  question  may  be 
attacked  by  certain  calculations  based  on  the  data  of  inter- 
ference. If,  as  a  rule,  chromosomes  twist  in  long  loops, 
crossing  over  at  two  points  close  together  would  be  rare, 
for  it  would  require  a  shorter  twist  than  usually  occurs. 
The  occurrence  of  long  loops  would  explain  the  interfer- 
ence of  neighboring  regions.  Moreover  the  decrease  of 
interference  as  distance  increases  would  be  accounted  for, 
because  short  loops  would  be  less  frequent  than  longer 
ones.  The  reappearance  of  interference  for  widely  separ- 
ated regions  is  explained  by  supposing  that  extremely 
long  loops  are  infrequent  as  are  very  short  ones.  That  is, 
on  the  supposition  of  long  twists  there  would  be  a  modal 
length  of  loop,  and  loops  of  greater  or  lesser  length  would 
be  less  frequent. 

If,  however,  the  chromosomes  are  tightly  twisted  into 
short  loops,  the  interference  of  neighboring  regions  might 
be  explained  on  the  supposition  that  a  break  at  one  point 
allows  the  chromosomes  partly  to  unravel  in  the  neighbor- 


INTERFERENCE  131 

hood  of  the  break,  and  that  this  loosens  the  twisting  and 
prevents  another  break  near  by.  In  regions  farther  away 
from  the  break,  the  threads  would  not  be  so  much  unrav- 
elled, so  that  the  greater  the  distance  from  the  first  point 
of  breaking  tlie  more  would  a  second  break  be  likely  to 
occur.  That  is,  interference  should  grow  less  at  greater 
distances.  But  the  reappearance  of  interference  at  still 
greater  distances  seems  incompatible  with  this  scheme; 
thus  the  actual  data  favor  the  first  view  of  crossing  over, 
in  which  the  break  occurs  during  a  stage  of  loose  twisting. 
At  any  rate,  as  Weinstein  has  pointed  out,  the  variation 
of  coincidence  with  distance  must  be  dependent  on  other 
conditions  than  the  mere  tension  due  to  the  twisting  of  the 
chromosomes,  and  any  view  which  refers  the  breakage  of 
the  threads  to  the  tension  of  tight  twisting  must  be 
rejected  or  supplemented. 

Castle  has  recently  suggested  that  the  difference 
between  the  values  for  a  long  ''distance''  and  summation 
of  short  ''distances''  is  due  to  the  loci  not  lying  in  a 
straight  line  but  "out  of  line."  He  suggested  that  when 
short  steps  are  taken  as  the  basis  for  map  distance 
they  represent  the  "long  way  round,"  as,  for  instance,  in 
passing  from  one  end  of  a  F  to  the  other  end,  keeping  on 
the  line;  while  when  a  direct  cross  is  made,  giving  a 
shorter  "distance,"  this  is  a  measure  of  the  direct  or 
air-line  between  the  two  ends  of  the  F.  Such  a  theory  is 
not  in  harmony  with  the  following  facts.  The  best  data 
(i.e.,  data  sufficient  in  amount  and  free  from  crossover 
variations)  show  that  Castle's  three  dimensional  fi,2:ures 
reduce  to  a  curved  line  in  a  plane.  In  such  a  curved  line 
the  most  distant  points  are  nearer  to  each  other  in  an  "  air- 
line" than  along  the  line.  Such  a  graphic  representation 
of  the  data  is  possible,  but  leads  to  certain  inconsistencies. 

If  Castle's  procedure  is  followed  it  leads  to  the  placini,' 
of  the  same  locus  in  two  or  more  different  places  on  the 
basis  of  adequate  and  comparable  data  for  both  positions. 
The  two  cases  that  Castle  says  furnish  the  crucial  evi- 


132  PHYSICAL  BASIS  OF  HEREDITY 

dence  for  his  view  demonstrate  just  the  opposite,  when 
complications  due  to  crossover  variations  are  excluded, 
by  using  only  data  in  which  three  or  more  loci  are  recorded 
simultaneously.  In  his  attempt  to  explain  the  all-import- 
ant fact  of  rarity  of  double  crossovers,  Castle  is  obliged 
to  assume  that  there  is  a  difference  in  frequency  of  cross- 
ing over  in  different  planes  (directions).  This  assump- 
tion can  be  shown  to  be  inconsistent  with  the  primary 
assumption  that  he  accepts,  viz.y  that  crossing  over  is 
proportional  to  the  distance  between  genes. 


CHAPTER  XI 
LIMITATION  OF  THE  LINKAGE  GROUPS 

It  may  be  questioned  whether  we  are  at  present  justi- 
fied in  speaking  of  the  limitation  of  the  linkage  groups 
to  the  number  of  chromosome  pairs  as  one  of  the  funda- 
mental principles  of  heredity,  since  the  only  species  in 
which  a  correspondence  that  is  numerically  significant 
between  the  two  has  been  proved  is  Drosophila  melano- 
gaster.  But  despite  the  absence  of  other  positive  evi- 
dence, the  fact  that  in  no  other  animal  or  plant  does  the 
number  of  linkage  groups  exceed  the  number  of  the 
chromosome  pairs,  may  be,  I  think,  legitimately  inter- 
preted in  favor  of  the  view. 

It  may  also  be  argued,  that  if  the  phenomena  of  linkage 
are  assumed  to  be  due  to  the  genes  being  carried  by  the 
chromosomes, it  follows  that  there  could  be  no  more  groups 
of  linked  genes  than  there  are  chromosome  pairs;  hence 
one  relation  is  the  direct  outcome  of  the  other.  But  the 
proof  of  the  linear  order  that  has  been  developed  here 
rests  directly  on  the  linkage  data,  and  is  independent  of 
any  assumption  concerning  the  chromosomes.  It  has  been 
shown,  secondarily  so  to  speak,  that  the  chromosomes  ful- 
fill all  the  requirements  of  the  abstract  reasoning  from 
the  data,  and  therefore  give  a  mechanism  capable  of  per- 
forming all  that  the  theory  demands.  The  demonstration, 
then,  that  in  Drosophila  the  linkage  groups  correspond 
in  number  to  the  chromosome  pairs  may  be  taken  as  a  con- 
clusion or  a  discovery  independent  of  the  other  relations 
furnished  by  linkage.  If  then,  as  I  anticipate  mil  be  the 
case,  further  work  in  other  groups  should  show  that  the 
same  relation  holds  everywhere,  we  should  be  fully  war- 
ranted in  stating  the  result  as  one  of  the  general  prin- 
ciples of  heredity. 

133 


134  PHYSICAL  BASIS  OF  HEEEDITY 

In  Drosophila  melanogaster  the  evidence  is  now  very 
strong  in  favor  of  the  identity  in  number  of  linkage  groups 
and  chromosome-pairs.  As  the  new  characters  coming  up, 
one  after  the  other,  have  continued  to  fall  into  the  four 
kno^\Ti  groups,  and  as  something  like  200  characters  have 
been  so  placed,  and  as  none  of  them  has  failed  to  show  link- 
age with  one  of  the  four  established  series,  the  probability 
is  enormously  in  favor  of  a  causal  relation  between  the  two 
events,  especially  in  the  light  of  the  evidence  from  other 
sources  that  the  chromosomes  are  the  bearers  of  the 
hereditary  factors — evidence  from  the  sex-chromosomes, 
for  example. 

The  only  other  species  in  which  the  heredity  of  known 
mutant  characters  approaches  that  of  the  chromosome 
group  is  the  garden  pea  in  which  about  35  mutant  factors 
have  been  studied.  From  the  summary  of  what  has 
been  so  far  recorded,  as  well  as  from  the  results  of  his 
own  work.  White  has  recently  given  an  account  of  what 
is  fairly  well  established.  Of  the  35  mutant  factors 
in  this  pea,  seven  independently  inherited  groups  have 
been  recorded,  i.e.,  each  one  of  seven  factors  has  been 
tested  out  and  found  to  assort  independently  of  the  other 
six.  There  are  seven  pairs  of  chromosomes  in  the  edible 
pea  (Fig.  53,  a).  The  agreement  between  the  two  is  to 
date  perfect.  It  is,  of  course,  possible  that  the  linkage 
between  some  of  the  factors  tested  was  so  loose  that  they 
a])peared  to  give  free  assortment,  and  that  until  more  fac- 
tors have  been  studied  the  evidence  is  not  above  sus- 
picion. Nevertheless,  it  is  important  to  find  that  the 
number  of  independent  mutant  factors  in  Pisum  sativum 
does  not  exceed  the  number  of  chromosome  pairs. 

White's  study  of  the  linkage  of  factors  in  the  edible 
peas  shows  further  that  there  are  four  linkage  groups-— 
three  of  them  include  factors  that  are  also  included  in 
those  that  freely  assort.  It  is  fair,  perhaps,  to  conclude 
that  four  of  the  possible  seven-linked  groups  have  been 
found.     There  are  no  other  forms  known  in  which  the 


LIMITATION  OF  THE  LINKAGE  GEOUPS  135 

number  of  linkage  groups  approaches  so  near  the  number 
of  the  chromosome  pairs.  In  the  snapdragon,  Baur  has 
described  two  linked  groups.  He  states  that  there  are  16 
pairs  of  chromosomes.  In  wheat  one  linked  group  has 
been  described.  There  are  8  pairs  of  chromosomes  (Fig. 
53,  h).  In  Indian  corn  there  appear  to  be  a  few  linkage 
groups,  and  probably  10  pairs  of  chromosomes.  In  oats, 
Surface  finds  two  linked  genes.  In  Primula  there  is  one 
group  composed  of  several  linked  genes,  and  12  pairs  of 
chromosomes  (Fig.  53,  c). 

In  the  silkworm  moth  one  linked  group  of  genes  has 
been  found  by  Tanaka,  and  Yatsu  records  (Fig.  55)  20 
pairs  of  chromosomes.  In  Drosophila  virilis  three  linked 
groups  of  genes  have  been  found  by  Metz,  who  has  also 


.f«r 


• 


Fig.  53. — Chromosome  group  of  pea,  a,   wheat,  b,   and  primula,  c. 

described  six  pairs  of  chromosomes  for  this  fly.  In  Droso- 
phila huscMi  there  is  one  group  of  linked  genes  and  four 
pairs  of  chromosomes.  In  D.  repleta  one  group,  and  six 
pairs  of  chromosomes.  The  groups  of  chromosomes  in 
some  of  the  different  species  of  Drosophila,  as  described 
by  Metz,  are  shown  in  Fig.  54.  As  indicated  by  the 
arrangement  of  the  figures  (that  correspond  fairly  closely 
with  the  actual  arrangement  of  the  chromosome  in  the 
cells  themselves)  it  appears  that  one  pair  of  chromosomes 
in  one  species  is  at  times  represented  by  two  pairs  in 
related  species,  and  this  view  is  borne  out  by  the  attach- 
ment of  the  spindle  fibre  to  the  middle  of  the  chromosomes 
in  the  bent  pairs,  but  to  the  inner  ends  of  the  two  that 
supposedly  correspond  to  its  halves  in  other  species. 

In  the  mouse  one  group  of  linked  genes  has  been 
reported.  There  are  20  pairs  of  chromosomes  ( Fig.  55,  b ) . 
In  man  no  linked  genes  are  known,  if  we  do  not  count  sex- 


136  PHYSICAL  BASIS  OF  HEREDITY 

lined  genes,  which  must,  however,  if  carried  by  the  sex- 
chromosomes,  be  linked  to  each  other.  The  number  of 
chromosome  pairs  in  man  is,  according  to  Grnyer,  12  (Fig. 
55,  c),  but  Winewarter  describes  24  pairs  (Fig.  55,  d). 
The  difference  would  seem  to  be  due  to  technic,  rather 
than  to  differences  in  different  races  of  men. 

Ma       IK       ih       IK 
IK       II.       IL      «„ 

^.^^  ^.^         M         II 


?    I   ^ 


ni 


i  J  <f 


Al 


•N?  K  ^/ll  <N^  ^   n| 


Fig.  54. — Types  of  chromosome  groups  found  in  Drosophila.  A-H  female  groups; 
T-L  female  and  male  groups.  In  A,  C,  F,  I,  J,  K,  and  L,  the  X-chromosome  can  be  identi- 
6ed,  because,  in  the  male  (Alex.  Metz),  the  Y-chromosome  has  a  different  shape  from  the  X. 

It  should  be  emphasized  that  it  is  to  be  expected  for 
new  types  that  the  number  of  characters  that  may  seem 
to  give  independent  assortment  will  be  found  at  first 
greater  than  the  number  of  chromosomes,  because  wher- 
ever two  genes  in  the  same  chromosome  are  far  apart  they 
will  appear  to  assort  independently  until  the  discovery 


LIMITATION  OF  THE  LINKAGE  GROUPS  137 


of  intermediate  genes  shows  their  true  relation.  This  will 
be  especially  the  case  when  crossing  over  occurs  in  both 
sexes ;  when  it  occurs  only  in  one  sex,  the  linkage  relations 
are  more  quickly  determined.  Moreover,  in  some  cases 
where  several  genes  are  known  the  mutant  characters 
have  not  yet  been  tested  out  against  each  other  but  against 
different  ones.  Such  information  does  not  furnish  the 
data  that  are  needed. 


a 


Fig.  55. — Haploid  group  of  chromosomes  of  the  silkworm  moth  (Yatsu)  a.  Haploid 
group  of  chromosomes  of  mouse  (Yocom)  b.  Haploid  group  of  chromosomes  of  man 
(Guyer),  c  and  (von  Winnewarter)  d. 

There  are  several  forms  in  which  there  are  two  or 
more  chromosomes  that  come  together  in  a  group  at  the 
time  of  segregation  and  move  collectively  to  one  pole. 
Such  groups  should  be  expected  to  count  as  a  single  chro- 
mosome so  far  as  segregation  is  concerned,  although  the 
crossing  over  relations  may  turn  out  to  be  something 
different  from  anything  as  yet  known. 


138  PHYSICAL  BASIS  OF  HEEEDITY 

An  extension  of  the  principle  of  agreement  of  linkage 
groups  and  chromosomes  (if  they  are  thought  of  only  as 
a  linear  order  of  genes)  is  found  in  the  case  of  ^'duplica- 
tion'^ described  by  Bridges,  where  a  short  series  of  linked 
genes  appears  to  lie  at  one  end  of  the  regular  series,  dupli- 
cating their  number  for  this  region  of  the  chromosome. 
Obviously  this  is  not  to  be  looked  upon  so  much  as  an 
exception  to  the  principle  but  rather  as  a  special  case  due 
to  an  accidental  change  in  the  mechanism.  The  number 
of  linkage  groups  is  not  changed,  but  one  of  them  has 
its  genes  duplicated  for  a  short  part  of  its  length. 


I 


CHAPTER  XII 
VARIATION  IN  LINKAGE 

Ceossing  over  is  not  absolutely  fixed  in  amount,  but 
is  variable.  This  statement  does  not  refer  to  variability 
in  the  number  of  crossovers  due  to  random  sampling, 
but  to  variability  due  to  fluctuation  in  environmental 
conditions,  or  due  to  internal  changes  in  the  mechanism  of 
crossing  over  itself.  For  example,  it  has  been  shown 
that  the  amount  of  crossing  over  in  Drosophila  is  different 
at  different  temperatures,  and  it  has  also  been  shown  that 
there  are  factors  (genes)  carried  by  the  chromosomes 
themselves  that  affect  the  amount  of  crossing  over.  These 
questions,  that  have  already  been  touched  upon  in  other 
connections,  may  be  taken  up  here  in  more  detail. 

The  work  of  Plough  on  the  influence  of  temperature 
on  crossing  over  in  Drosophila,  that  has  already  been 
utilized,  was  concerned  with  the  influence  of  different  tem- 
peratures on  the  number  of  crossovers  obtained.  It  may 
be  recalled  that  he  found  that  when  the  eggs  were  sub- 
jected to  a  given  temperature  during  a  certain  stage  in 
their  maturation  the  amount  of  crossing  over  that  took 
place,  as  shown  in  the  kinds  of  flies  produced,  was  definite 
in  the  sense  that  the  average  results  were  predictable  for 
each  specific  temperature,  and  that  there  are  values  for 
different  temperatures  which,  when  plotted,  give  the  curve 
drawn  in  Fig.  56. 

Further  details  of  one  of  the  experiments  may  serve 
to  make  its  significance  clearer.  Three  points  (or  loci) 
were  made  use  of  that  involved  three  mutant  genes  (and 
their  diagnostic  charaqters,  of  course).  Males,  pure  for 
the  three  mutant  characters,  black  body  color,  purple  eyes, 

139 


140 


PHYSICAL  BASIS  OF  HEREDITY 


and  curved  wings  were  crossed  to  wild-type  females.  The 
Fj  female  produced  in  this  way  would  be  heterozy- 
gous for  the  three  mutant  factors  involved  in  the  cross. 
Such  an  i^j  female  was  then  bred  to  a  male  pure  for  the 
three  recessive  genes,  black,  purple,  cui'A^ed;  and  her 
offspring  were  kept  at  a  given  temperature  until  they 
emerged  as  flies,  and  then  if  necessary  for  some  days 
longer  in  order  that  as  many  eggs  as  possible  might  have 
matured  under  the  specified  temperature.  Controls  of 
sisters  and  brothers  were  made  in  each  case  and  kept  at 
average  ^'normal"  temperature.  In  the  table  that  fol- 
lows crossing  over  between  black  and  purple  is  indicated 
as  ^^Ist  crossover,''  and  between  purple  and  curved  as 
^*2nd  crossover,''  and  between  both  as  double  crossover. 
Ten  different  temperatures  were  tested.  At  5°  C.  the 
eggs  did  not  hatch,  and  at  35°  C.  the  females  were  sterile. 
In  the  seven  intermediate  temperatures  the  results  were 
those  recorded  in  the  next  table. 


b  —  pr  —  c^ 


Female  parents  hatched  at  te 

mperature 

indicated  below 

Weighted 

Temp. 

Total 

Value  for 

Num- 

Non- 

1st 

2nd 

Double 

ISt 

2nd 

b  — pr 

ber 

cross- 

cross- 

cross- 

cross- 

cross- 

cross- 

Region 

over 

over 

over 

over 

over 

over 

per  cent 

per  cent 

per  cent 

2 

r 

995 

643 

95 

218 

39 

13.5 

25.8 

13.6 

3 

13° 

2,972 

1,854 

310 

716 

92 

13.5 

27.2 

17.5 

4 

17.5° 

2,870 

2,021 

189 

610 

50 

8.3 

23.0 

8.2 

5 

22° 

15,000 

11,318 

735 

2,775 

172 

6.0 

19.6 

6.0 

7 

29° 

4,269 

2,993 

315 

898 

63 

8.8 

22.5 

8.7 

8 

31° 

3,547 

2,265 

333 

785 

164 

14.0 

26.7 

18.2 

9 

32° 

4,376 

2,701 

513 

984 

178 

15.7 

26.5 

15.4 

At  the  two  lower  temperatures  the  crossover  value  is 
high,  i.e.^  little  crossing  over  occurs.  At  the  next  three 
temperatures  (17.5°,  22°,  29°  C.)  the  crossing  over  value 
is  much  less,  while  at  the  last  two  temperatures  29°  and 


VARIATION  IN  LINKAGE  141 

31°  C,  it  is  high  again.  The  control  values  for  sister 
flies,  at  normal  temperature  (22°  C),  are  given  in  the 
next  table. 


Controls — female  parents  hatched  at  22°  C. 


I  St 

cross- 
over 

2nd 
cross- 
over 

Total 

Non- 
cross- 
over 

1st 
cross- 
over 

2nd 
cross- 
over 

Double 
cross- 
over 

per  cent 

per  cent 

6.1 

7.8 
5.9 

19.2 
20.1 
19.5 

904 
3,622 
2,219 

683 
2,655 
1,678 

47 
231 
108 

166 
685 
409 

8 
51 
24 

5.9 

20^3 

4,822 

3,608 

231 

927 

'56* 

.... 

.... 

.... 

•  •    •  • 

.... 

.... 

.... 

The  figures  given  in  this  table  were  obtained  as  a  con- 
trol for  the  last  results,  and  from  these  data  the  results 
of  crossing  over  are  reduced  to  the  same  scale.  These 
weighted  crossing-over  values  for  the  first  regions  give 
the  curve  drawn  in  Fig.  56.  The  curve  begins  at  a  high 
level  and  drops  rapidly.  The  first  maximum  is  reached  at 
about  13°  C,  and  then  falls  to  17.5°  C,  where  the  level 
remains  nearly  constant  for  ten  degrees  more  (27°  C). 
It  rises  rapidly  at  about  28°  and  reaches  a  second  maxi- 
mum at  31°  to  32°  C.  Afterwards  it  is  seen  to  fall  until 
sterility  occurs  at  35°  C. 

The  temperature  curve  of  crossing  over  seems  to  show 
that  the  phenomenon  is  not  a  simple  chemical  reaction, 
for  if  it  were  we  should  expect  for  every  rise  in  10°  C.  the 
amount  of  change  in  crossing  over  to  be  approximately 
tripled.  It  would  appear,  therefore,  that  the  phenomena 
might  be  due  to  the  physical  state  of  the  materials  involved 
in  crossing  over.  Plough  calls  attention  to  the  similarity 
of  this  curve  to  that  shown  by  the  amount  of  contraction 
of  a  frog's  muscle.  Here  there  is  an  increase  from  zero 
to  9°  C,  when  a  maximum  is  reached.  After  this,  the 
amount  of  contraction  decreases,  reaching  a  low  point 


142 


PHYSICAL  BASIS  OF  HEREDITY 


between  10°  C.  and  20°  C.  It  then  rises  rapidly,  reaching 
a  higher  maximum  than  the  first  at  about  28°  C,  after 
which  it  decreases  until  rigor  sets  in  at  38°  C. 

The  results  of  crossing  over  between  purple  and  curved 
gave  similar  results,  but  the  ** distance''  here  is  so  great 
that  double  crossing  over  complicates  the  results;  there- 
fore they  need  not,  for  the  present,  be  analyzed  further. 
Attempts  to  change  the  crossing  over  value  by  starvation, 
moisture,  increase  in  fermentation  of  the  food,  iron  salts, 
etc.,  gave  no  results  that  seemed  significant.    On  the  other 


% 

« 
II 


Of 


♦ 

t 

D«fi'<«»  C 

* 

IS 

ITJ 

u 

27      2«      31  32 

1 

■ 

Fig.  56. — Curve  showing  influence  of  crossing  over  at  different  temperatures.  (After  Plough.) 

hand,  Bridges  had  already  noted  that  a  decrease  in  the 
amount  of  crossing  over  is  found  in  second  broods  as 
compared  with  first  broods — ten-day  periods.  What 
change  in  the  environment  is  behind  this  ^^age''  dif- 
ference is  not  clear,  but  since  most  of  the  eggs  pass 
through  this  early  prematuration  stage  in  the  larvaB 
and  some  of  them  may  reach  the  maturation  stage 
in  the  pupa,  it  is  possible  that  prevailing  conditions  in 
one  or  the  other  of  these  physiological  states  may  be 
responsible  for  the  difference  between  these  states  and 
those  that  prevail  after  the  fly  has  hatched. 


VARIATION  IN  LINKAGE  143 

Not  only  external  factors  but  internal  factors,  and 
these  genetic  ones,  may  influence  the  amount  of  crossing 
over  that  takes  place.  Sturtevant  has  discovered  two  such 
genes  in  the  second  chromosome  of  a  certain  stock  of 
Drosophila.  A  female  from  a  wild  stock  from  Nova 
Scotia  was  crossed  to  a  male  showing  the  characters  ves- 
tigial and  speck.  One  of  the  daughters  was  tested  and 
gave  no  crossovers  in  99  offspring,  though  the  vestigial, 
speck  hybrid  usually  gives  about  37  per  cent,  of  crossing 
over.     All  of  the  descendants  of  this  female  that  were 

^^ ^  If Y  C Bp 


0«  37.9        44.1  55.9      6*.0 


iti.2 


ab  a'2  1.3  £4 


o.'o  0:6 

♦- \ i^f 

0.0  42.4     46.6  •.-.•-.. 


13.4         21.0  56.3 


^■^ ^^ 

OJD  03  014 


"aSTua 


»P 


0.0  3^2  4L7  M-2 

FiQ.  57. — Diagram  illustrating  the  effect  on  crossing  over  due  to  the  presence  of  crossover 

genes.    (After  Sturtevant.) 

known,  through  linkage  relations,  to  have  the  Nova  Scotia 
second  chromosome,  gave  the  same  result,  while  those  of 
her  descendants  that  did  not  have  the  particular  chromo- 
some did  not  show  such  a  change  in  linkage.  These  rela 
tions  held  regardless  of  whether  the  chromosome  involved 
had  come  from  the  father  or  the  mother. 

A  number  of  experiments  were  made  with  females  hav- 
ing a  Nova  Scotia  second  chromosome,  while  the  other 
second  chromosome  bore  the  mutant  genes  for  black,  pur- 
ple, curved,  and  in  other  experiments  other  mutant  genes 
were  present.  In  Fig.  57  (upper  line)  all  the  genes  stud- 
ied, viz.,  star  (S)^  black  (fc),  purple  (pr),  vestigial  (vg), 


144  PHYSICAL  BASIS  OF. HEREDITY 

curved  (c),  and  speck  (sp)  are  indicated  in  their  relative 
locations,  i.e.,  spaced  in  proportion  to  the  usual  amount 
of  crossing  over  between  them.  Correspondingly,  the 
short  second  line  is  based  on  the  crossover  relations  of 
these  factors  when  the  female  is  heterozygous  for  the  two 
Nova  Scotia  genes. 

Further  experiments  were  made  with  females 
(obtained  by  crossing  over)  in  which  only  the  "left  half '^ 
of  a  Nova  Scotia  chromosome  was  present  (third  line), 
the  other  half  being  derived  from  an  ordinary  chromo- 
some. The  offspring  of  such  a  female  showed  that  cross- 
ing over  was  decreased  only  in  the  left  half. 

When  the  right  half  of  the  Nova  Scotia  chromosome 
was  present  (fourth  line)  that  half  was  "shortened.'^  It 
follows  that  there  are  two  (or  possibly  more)  factors 
present,  one  in  each  half  of  the  second  chromosome  of  the 
Nova  Scotia  stock,  each  inhibiting  almost  completely 
crossing  over  in  its  own  region,  but  not  in  the  other  region. 

An  equally  surprising  result  was  obtained  from  a 
female  so  constituted  that  the  right  halves  of  both  mem- 
bers of  this  pair  of  second  chromosomes  were  present,  ke., 
when  she  was  homozygous  for  the  "right  hand^'  pair  of 
factors  for  little  crossing  over.  Under  these  circum- 
stances, the  crossing  over  was  normal  for  this  end  (last 
two  lines).  How  such  results  are  produced  (quite  aside 
from  the  nature  of  the  factor  producing  them)  is  unknown. 
Almost  inevitably,  however,  we  think  of  the  cause  as  a 
difference  in  the  length  or  shape  of  the  chromosome  con- 
taining these  factors,  so  that  corresponding  levels  do  not 
come  together,  hence  failure  of  interchange.  When,  how- 
ever, both  chromosomes  are  affected  in  the  same  way  their 
corresponding  regions  might  be  expected  to  come  to- 
gether and  cross  over. 

The  preceding  results  of  Sturtevant's  suggest  the 
possibility  that  all  genes  may  have  an  effect  on  crossing 
over — possibly  one  might  think  that  in  some  mysterious 
way  the  crossing-over  values  shown  by  the  genes  are  a 


VARIATION  IN  LINKAQE  145 

function  of  their  nature.  It  may  be  well  to  point  out  that 
in  the  only  cases  where  the  evidence  suffices  to  give  an 
answer  to  such  a  question,  that  answer  is  very  clearly 
against  such  a  view.  For  instance,  if  we  determine  the 
linkage  between  two  factors  A-M  and  then  exchange  one 
of  the  intermediate  genes  for  its  allelomorph,  we  find  that 
in  general  the  change  has  no  effect  on  crossing  over 
between  A  and  M.  If  we  exchange  factors  outside  of 
A  and  M — either  near  them  or  far  away — still  no  effect  on 
crossing  over  between  A  and  M  is  observed.  K  we  sub- 
stitute one  allelomorph  for  another,  in  cases  where  more 
than  two  are  known,  we  find  no  change  in  the  crossing 
over  for  that  level.  This  and  other  evidence  shows  that 
crossing  over  is  quite  independent  of  such  genes,  never- 
theless there  are  other  specific  genes,  as  shown  above, 
whose  sole  effect,  or  main  effect  at  least,  is  to  change  the 
crossing-over  values. 

One  highly  important  and  significant  result  of  Sturte- 
vant's  work  on  crossing-over  factors  should  be  noticed. 
The  order  of  the  factors  is  not  in  any  way  changed  by 
the  ^^ shortening"  process,  as  shown  by  the  experiments 
in  which  three  or  more  loci  are  followed  at  the  same  time. 

The  most  remarkable  fact  connected  with  crossing 
over  is  that  no  crossing  over  at  all  takes  place  in  the 
male  of  Drosophila,  and  this  applies  not  only  to  sex- 
chromosomes  (XY)  but  also  to  the  other  pairs  or  auto- 
somes. When  the  absence  of  crossing  over  was  discovered 
for  sex-linked  genes,  it  seemed  probable  that  this  was  due 
to  the  presence  of  only  one  X-chromosome  in  the  male,  for 
at  this  time  Steven's  work  had  led  us  to  conclude  that  the 
male  Drosophila,  like  some  other  insects,  is  XO.  Later, 
when  failure  to  cross  over  in  the  male  was  found  in  other 
chromosomes  as  well,  it  was  evident  that  some  more  gen- 
eral relation  was  behind  the  phenomenon  in  these  chromo- 
somes at  least.  It  is  true  that  other  genetic  evidence 
has  shown  that  the  F-chromosome  is  ''empty''  {i.e.,  con- 
tains no  genes  dominant  to  any  of  the  mutant  genes  as  yet 

10 


146  PHYSICAL  BASIS  OF  HEREDITY 

discovered)  and  on  this  account  one  might  still  ascribe 
failure  to  cross  over  in  this  pair  to  its  peculiar  condition. 

The  interest  in  the  situation  became  even  greater  when 
it  was  found  that  in  the  silkworm  moth  (in  which  the  sex 
formula  is  reversed,  so  to  speak)  crossing  over  is  again 
absent  in  the  sex  that  is  heterozygous  for  the  sex  fac- 
tors— here  the  female.  The  female  moth  is  apparently 
ZW,  at  least  in  two  cases. 

In  one  of  the  flowering  plants,  Primula  sinensis,  cross- 
ing over  occurs  in  both  sexes  (Gregory,  Altenburg),  but 
the  amount  of  crossing  over  in  the  pollen  is  somewhat  dif- 
ferent from  that  in  the  ovules.  Gowen  has  examined 
Altenburg 's  data  statistically  and  finds  that  the  differ- 
ence is  probably  significant. 

That  crossing  over  should  take  place  in  the  sex  that  is 
homozygous  for  the  sex-chromosomes  (the  female  in 
Drosophila,  the  male  in  the  silkworms)  but  in  both  sexual 
elements  in  the  hermaphrodite  plant  (Primula)  may 
appear  to  have  a  deeper  significance,  but  more  recent  dis- 
coveries seem  to  deprive  the  results  of  any  such  meaning. 
Castle,  for  instance,  gives  data  that  show  crossing  over 
in  the  male  rat  (the  male  is  probably  heterozygous  for 
the  sex-chromosome) ,  and  Nabours  gives  data  for  crossing 
over  in  the  male  and  female  grouse  locust,  Apotettix 
(in  which  the  male  is  presumably  heterozygous).  Until 
more  cases  are  forthcoming  it  must  seem  doubtful,  there- 
fore, if  any  such  relation  as  that  mentioned  above  is  a 
general  one. 


CHAPTER  XIII 

VARIATION  IN  THE  NUMBER  OF  THE  CHROMO- 
SOMES AND  ITS  RELATION  TO  THE  TOTAL- 
ITY OF  THE  GENES 

The  theory  that  the  chromosomes  are  made  up  of  inde- 
pendent self-perpetuating  elements  or  genes  that  compose 
the  entire  hereditary  complex  of  the  race,  and  the  impli- 
cation contained  in  the  theory  that  similar  species  have  an 
immense  number  of  genes  in  common,  makes  the  numeri- 
cal relation  of  the  chromosomes  in  such  species  of  un- 
usual interest.  This  subject  is  one  that  could  best  be 
studied  by  intercrossing  similar  species  with  different 
numbers  of  chromosomes,  but  since  this  would  yield  sig- 
nificant results  only  in  groups  where  the  contents  of  the 
chromosomes  involved  were  sufficiently  known  to  follow 
their  histories,  and  since  as  yet  no  such  hybridizations 
have  been  made,  we  can  only  fall  back  on  the  cytological 
possibilities  involved,  and  on  the  suggestive  results  that 
cytologists  have  already  obtained  along  these  lines. 

A  good  deal  of  attention  has  been  paid  in  recent  years 
to  the  not  uncommon  fact  that  one  species  may  have 
twice  as  many  chromosomes  as  a  closely  related  one.  So 
frequent  is  this  occurrence  that  it  seems  scarcely  possible 
that  it  is  due  to  chance.  The  implication  is  that  the  num- 
ber of  the  original  chromosomes  has  either  become 
doubled,  or  else  halved.  If  the  number  is  simply  doubled 
there  would  be  at  first  four  of  each  kind  of  chromosome 
from  the  point  of  view  of  genetic  contents.  This  is  what 
I  understand  by  tetraploidy.  There  is  some  direct  evi- 
dence that  doubling  may  occur.  If  a  new  race  or  species 
is  ever  established  in  this  way,  we  should  anticipate  that 
in  the  course  of  time  changes  might  occur  in  the  four  iden- 
tical chromosome  groups  so  that  they  would  come  to  differ 

147 


148  PHYSICAL  BASIS  OF  HEREDITY 

and  form  two  different  sets.^  Theoretically,  the  number 
of  different  genes  in  a  species  might  in  this  way  be  in- 
creased. If  changes  in  the  same  gene  in  the  same  direction 
sometimes  occur,  as  the  evidence  indicates  that  they  do, 
then  identical  new  mutant  genes,  derived  from  the  same 
kind  of  original  ones,  might  later  arise  in  different  pairs. 

There  is,  however,  another  way  in  which  the  number 
of  chromosomes  may  be  doubled  without  doubling  the 
number  of  genes.  If  the  chromosomes  break  in  two, 
double  the  number  will  be  produced.  It  is  not  easy  to 
explain  how  this  could  occur  in  all  of  the  chromosomes  at 
the  same  time  if  the  process  is  supposed  to  be  accidental. 
If  it  be  supposed  that  the  break  first  occurred  accidentally 
in  one  member  of  the  pair,  it  is  not  clear  why  such  a 
broken  chromosome  could  establish  itself  on  the  theorv 
of  chance,  for  the  intermediate  condition  of  one  broken 
and  one  intact  chromosome  would  seem  of  no  apparent 
advantage.  The  same  reasoning  applies  to  the  converse 
process,  vis.,  the  coming  together  of  chromosomes  end 
to  end  which  would  reduce  the  number  by  half.  Such  a 
process  would  not  increase  the  number  of  genes  in 
the  total  complex.  Until  we  know  more  about  the 
physical  or  chemical  forces  that  hold  the  genes  in  chains, 
and  more  about  the  way  new  genes  arise,  it  is  not  worth 
while  to  speculate  about  the  causes  or  probabilities 
of  such  occurrences. 

What  has  just  been  said  in  regard  to  doubling  and 
halving  of  the  whole  set  of  chromosomes  applies  also  to 
doubling  in  one  pair  of  chromosomes.  If  doubling 
occurred  in  one  pair  of  a  ten-chromosome  type,  a  twelve- 
chromosome  type  would  result ;  if  in  two  pairs,  a  f  ourteen- 
chromosome  type,  etc.  Unless  tetraploidy  is  the  simpler 
procedure  we  should  a  priori  suppose  that  increasing  (or 
decreasing)  in  pairs  would,  on  the  theory  of  chance  alone, 

^  The  question  as  to  whether  the  four  chromosomes  involved  would  or 
would  not  mate  at  random  introduces  a  difficulty  (as  sho^vn  in  the 
primula  case). 


VARIATION  OF  CHROMOSOMES  149 

be  the  more  common  procedure.  A  few  examples  will 
illustrate  what  has  been  found  out  so  far  concerning  some 
of  these  possibilities. 

The  evening  primrose,  (Enothera  lamarckicma,  has  14 
chromosomes  as  its  full  or  somatic  number,  and  7  as  its 
reduced  number  (Fig.  58,  a),  and  these  numbers  charac- 
terize most  of  the  mutant  types  that  De  Vries  found.  But 
there  is  one  mutant  known  as  gigas,  that  has  28  chromo- 
somes as  its  full  number,  and  14  as  its  reduced  number 
(Fig.  58,  &).  Stomps  estimates  that  gigas  appears  about 
9  times  in  a  million  cases,  i.e.,  in  0.0009  per  cent.  G-igas  is 
distinguished  from  LamarcJciana  in  many  details  of  struc- 
ture, but  chiefly  in  its  thick  stem,  etc.,  which  is  associated 
with  larger  cells. 


:&\s  -^m  ^^ 


ad  c 

Fia.  5S. — Chromosome  group  of  CEnothera   lamarcJciana,  a;    chromosome  of    group   of  0. 

gigas,  b;   triploid  group,  c. 

The  type  breeds  true,  i.e. At  does  not  revert  to  Lamarck- 
iana;  thus  De  Vries  grew  a  family  of  450  individuals  from 
his  original  gigas,  only  one  being  a  dwarf  gigas,  viz., 
nanella.  The  way  in  which  gigas  originates  has  been 
much  discussed,  but  no  conclusion  reached.  De  Vries 
suggested  that  it  is  produced  by  an  egg  with  14  chromo- 
somes (diploid),  being  fertilized  by  a  sperm  with  14 
chromosomes,  both  of  these  diploid  cells  originating  by 
the  suppression  of  a  cytoplasmic  division  in  the  develop- 
ment of  the  gametes.  It  has  also  been  suggested  that  a 
tetraploid  condition  might  arise  in  a  spore  mother  cell 
that  developed  without  fertilization  (by  apospoiy ) .  Gates 
pointed  out  that  by  suppression  of  the  first  division  of  the 
egg,  after  fertilization,  the  tetraploid  condition  would 
arise.     The  only  objection  to  this  last  view,  that  seems 


150  PHYSICAL  BASIS  OF  HEREDITY 

the  simplest  one  since  such  suppressed  division  has  been 
seen  and  can  be  induced  in  animal  eggs,  is  that  the  follow- 
ing division  might  be  expected  to  be  into  four  parts  owing 
to  the  doubling  of  the  centres. 

Gregory  has  described  two  tetraploid  races  of  Primula 
sinensis,^  one  of  which  arose  from  ordinary  plants  in  the 
course  of  his  experiments.  Since  known  genetic  factors 
were  present  an  opportunity  was  given  to  examine  into  the 
relation  between  the  members  of  the  four  chromosomes 
of  a  set.  The  possibilities  involved  are  these :  Assuming 
the  parents  to  be  AA\  and  aa\  and  that  conjugation  of 
chromosomes  takes  place  in  twos  only,  then  if  any  one 
of  the  four  {A A'  aa')  chromosomes  of  a  set  may  mate  with 
any  other  member,  there  will  be  six  possible  unions,  viz., 
AA',  Aa,  Aa',  A' a.  A' a',  aa'.  If  the  two  derived  from 
the  same  parents  were  the  only  ones  that  can  mate,  only 
two  combinations  are  possible,  AA',  aa',  and  if  the  two 
derived  from  the  opposite  parents  were  the  only  ones  that 
mate  only  two  (but  different  ones)  could  form,  viz.,  Aa, 
A' a'.  The  genetic  expectation  is  somewhat  different 
for  each  of  the  three  cases,  since  the  number  of  different 
kinds  of  gametes  produced  is  different  in  each.  The  data 
obtained  by  Gregory  are  not  sufficient  to  give  convincing 
evidence  in  favor  of  any  one  of  these  possibilities,  although 
as  Muller  has  shown  by  an  analysis  of  the  evidence,  they 
are  more  in  favor  of  the  first  possibility,  viz.,  that  of  ran- 
dom assortment.  Gregory,  without  coimnitting  himself  to 
the  chromosome  view,  follows  the  second  possibility  in  his 
analysis  of  the  case.  There  is,  however,  nothing  in  the 
chromosome  theory  that  would  support  the  view  that 
restricts  the  conjugation  of  homologous  chromosomes 
according  to  their  parental  origins. 

There  are  two  other  species  of  primose,  Primula  fiori- 
hunda  and  P.  verticillata,  each  with  18  chromosomes  that 
have,  after  crossing,  produced  tetraploid  types.     In  a 

*  other  giant  races  of  P.  sinensis  examined  by  Keeble  and  by  Gregory 
are  diploid. 


i 


VARIATION  OF  CHROMOSOMES  151 

cross  between  these  two,  a  hybrid  called  P.  keivensis  was 
produced,  which  Digby  has  shown  has  also  18  chromo- 
somes. It  produced  only  thrum  flowers,  and  was  therefore 
sterile.  Five  years  later,  after  this  plant  had  been  multi- 
plied by  cuttings,  one  pin  flower  appeared  which  was  pol- 
linated by  a  thrum  flower.  It  gave  rise  to  the  fertile  race 
of  P.  keivensis,  that  had  36  chromosomes.  What  connec- 
tion there  may  have  been  between  the  hybridization  and 
the  subsequent  doubling,  if  there  is  any  connection,  is  by 
no  means  clear.  It  may  be  noted  that  in  the  reciprocal 
cross  between  P.  verticillata  and  P.  floribunda,  a  hybrid, 
P.  kewensis,  with  36  chromosomes  also  appeared. 

The  most  interesting  results  on  tetraploidy  are  those 
of  Elie  and  Emile  Marchal  on  certain  mosses,  for  they 
have  been  able  to  produce  tetraploid  types  experimentally. 
It  may  be  recalled  that  in  mosses  there  is  an  alternation 
of  generations.  The  diploid  (22V)  generation  is  known 
as  the  sporophyte  (Fig.  59)  that  develops  out  of  and 
remains  attached  to  the  other  haploid  generation,  the 
gametophyte  or  moss  plant  (IN).  The  sporophyte  pro- 
duces a  large  number  of  spores,  each  containing  the  half 
number  of  chromosomes  (IN)  as  a  result  of  reduction  that 
has  taken  place  in  their  formation,  and  from  each  spore 
a  young  moss  plant  develops,  beginning  as  a  protonema  of 
loose  threads.  When  the  moss  plant  produces  its  heads  or 
flowers  the  sexual  organs  appear — archegonia  (  9  )  and 
antheridia  (  S).  Thus  the  ^'sexes''  are  here  represented 
by  the  haploid  generation. 

The  egg-cell,  contained  in  the  archegonium,  is  ferti- 
lized by  a  sperm-cell,  the  antherozooid.  The  fertilized 
egg-celi  {2N)  develops  in  situ  into  the  straight  stalk 
imbedded  at  its  lower  end  in  the  tissue  of  the  moss  plant, 
expanding  at  its  upper  end  into  the  cup  containing  the 
spores.  The  mother-cells  of  the  spores — like  the  tissue  of 
the  sporophyte  itself — contain  the  2N  number  of  chromo- 
somes, which,  by  two  divisions  (similar  to  these  already 
described  for  the  animal  cells  during  reduction),  reduces 


152 


PHYSICAL  BASIS  OF  HEREDITY 


the  number  to  IN.  It  is  at  this  time,  too,  in  mosses  with 
separate  sexes,  that  sex  differentiation  takes  place,  for  as 
the  Marchals  have  shown,  each  spore  gives  rise  to  a  male 


®      ®  XiTl) 


xm) 


xm.) 


X  y  i2n)  Sporophyie 


X(Ti)       Qametophyk 


©xm) 


FiQ.  59. — Life  cycle  of  moss.  The  mycelial  thread  and  the  moss  plant  constitute 
the  In,  or  gametophyte  generation;  and  the  stalk  and  capsule  (with  its  contained  spores), 
arising  after  fertilization  out  of  the  moss  plant,  constitutes  the  Sn  or  sporophyte  generation. 

or  to  a  female  thread  that  produces  archegonia  or  else 
antheridia  regardless  of  the  condition  under  which  the 
young  plants  are  reared.  Allen  has  recently  shown  in 
related  plants — the  liverworts — that  during  the  reduction 
division  (that  gives  rise  to  the  spores)  an  unpaired  sex- 


VARIATION  OF  CimOMOSOMES 


153 


chromosome  is  present  that  goes  to  half  only  of  the  spores. 
Presumably  then  in  liverworts,  and  mosses,  also,  there 
is  an  internal  mechanism  for  producing  the  two  ''sexes." 
The  Marchals  have  worked  both  with  species  having 
separate  sexes  and  with  hermaphrodites.    We  may  con- 


xyizn) 


y(27U 


>T/m)    ®  x(nt 


Fig.  60. 


xy(4n) 


Fxa.  60. — Diagram  illustrating  the  formation  of  £n  individuals  from  the  regeneration  of 
the  sporophyte  in  a  dioecious  species.     (According  to  Marchal.) 

Fia.  61. — Diagram  illustrating  the  formation  of  2n  individuals  from  the  regeneration  of 
the  sporophyte  in  a  hermaphroditic  species.     (According  to  Marchal.) 

sider  the  former  first.  If  the  sporophyte  is  removed  and 
cut  across,  its  cells  regenerate  a  tangle  of  threads  (pro- 
tonema),  which  is  the  beginning  of  a  new  moss  plant  (Fig. 
60).  Since  the  sporophyte  had  the  double  number  (2iY) 
of  chromosomes,  it  is  to  be  expected  that  the  young  moss 
plant  that  regenerates  from  its  tissue  (sporophyte)  will 
also  have  the  double  number,  and  such  proves  to  be  the 


154  PHYSICAL  BASIS  OF  HEREDITY 

case.  The  new  moss-plant  is  therefore  2N  (or  diploid) 
instead  of  being  liV,  as  in  the  normal  mode  of  propaga- 
tion. Since  no  reduction  has  taken  place  into  male-  and 
female-producing  individuals,  it  would  seem  possible  that 
such  a  plant  might  produce  either  or  both  sexes.  Such 
is  the  case,  for  Avhen  the  2N  moss  plant  produces  its 
*' flowers^'  some  contain  archegonia,  others  spermato- 
gonia (with  their  contained  germ-cells)  and  other  flowers 
contain  both.  The  hermaphroditism  here  produced  would 
seem  to  be  the  sum  of  both  the  contrasted  elements.  The 
expectation  from  such  a  2N'  plant  would  be  that  its  germ- 
cells  i2N)  would  produce  a  4iV  sporophyte — unfortunately 
the  plants  proved  sterile.  Imperfect  germ-cells  were 
present  incapable  of  fertilizing  or  of  being  fertilized, 
so  that  it  was  not  possible  to  perpetuate  the  2A^  plant  by 
sexual  reproduction. 

The  results  mth  the  2N  plants  derived  from  the  regen- 
erating sporophyte  of  the  hermaphroditic  species  (Fig. 
61)  is  different  in  one  important  respect.  When,  as 
before,  a  diploid  (2N)  plant  is  obtained  by  regeneration 
from  the  sporophyte  it  produces  hermaphroditic  flowers, 
i.e.,  flowers  containing  both  oogonia  and  spermatogonia, 
and  these  are  fertile.  The  sporophyte  that  they  produce 
is  tetraploid  (4iV),  due  to  the  union  of  a  diploid  anther- 
ozooid  with  diploid  egg.  Regeneration  from  the  tetraploid 
sporophyte  (4iV)  should  produce  fertile  gametes,  which 
might  give  rise  by  their  union  to  an  octoploid  sporophyte 
(SN).  So  far  the  Marechals  have  not  been  able  to  produce 
such  plants,  for  although  in  a  few  cases  the  4:N  sporophyte 
regenerated  it  failed  to  produce  flowers. 

The  difference  then  between  the  results  from  mosses 
with  separate  sexes  and  mosses  that  are  hermaphrodite  is 
that  the  2N  plant  of  a  race  with  separate  sexes  does  not 
form  normal  gametes,  while  a  2N  plant  of  hermaphroditic 
races  forms  fertile  gametes.  It  may  appear  more  or  less 
plausible  that  the  failure  of  the  former  is  due  to  failure 
in  the  reduction  of  the  spores  into  two  alternative  types, 


VARIATION  OF  CHROMOSOMES  155 

while  in  the  latter  case,  since  there  are  presumably  no 
such  types  found,  there  is  no  conflict.  Some  other  dif- 
ference would  have  to  be  appealed  to  to  explain  why  the 
octoploid  forms  fail  to  develop. 

A  triploid  condition  {3N)  has  been  found  to  occur  in 
certain  types  of  the  evening  primrose  (Stomps,  Lutz, 
Gates).  De  Vries  has  found  in  crosses  in  which  Lamarck- 
iana  was  the  mother  and  some  other  species  (muricata, 
cruciata,  etc.),  the  father,  that  triploid  types  appear  three 
times  in  1000  cases.  He  interprets  the  results  to  mean 
that  three  in  1000  times  the  egg-cell  of  Lamarchiana  has 
the  double  number  of  chromosomes  (14),  which  being  fer- 
tilized by  a  normal  pollen  grain  with  seven  chromosomes, 
gives  the  triploid  number,  viz.,  twenty-one  chromosomes. 
The  same  result  would  be  reached  if  a  diploid  pollen  grain 
fertilized  a  normal  Qgg,  That  such  pollen  grains  appear 
is  as  probable  a  priori  as  that  diploid  eggs  occur.  It 
may  be  recalled  that  one  explanation  of  the  tetraploid 
evening  primrose  (gigas)  is  that  it  arises  from  a  22V  pollen 
grain  meeting  a  2N  egg-cell.  How  reduction  takes  place 
in  the  triploid  Oenotheras  is  uncertain,  since  the  accounts 
of  the  process  are  different.  Geerts  states  that,  as  a  rule, 
only  seven  chromosomes  conjugate  (7  +  7),  while  the 
remaining  seven  chromosomes  are  irregularly  distrib- 
uted in  the  dividing  germ-cells.  On  the  other  hand,  Gates 
finds  in  a  21-chromosome  type  that  the  chromosomes 
separate  into  groups  of  10  and  11,  or  occasionally  into 
9  and  12.  The  former  account  fits  in  better  with  results 
of  the  same  kind  obtained  by  others,  and  is  more  easily 
understood  from  a  general  point  of  view,  because  seven 
homologous  pairs  would  correspond  to  the  normal  conju- 
gation, while  the  seven  chromosomes  left  over  would  have 
no  mates  and  fail  to  divide  at  the  reduction  division,  hence 
their  erratic  distribution. 

It  has  also  been  shown  in  (Enothera  that  there  are 
three  15-chromosome  types.    If  the  15th  chromosome  is 


156 


PHYSICAL  BASIS  OF  HEREDITY 


sometimes  one,  sometimes  another  chromosome,  there  may 
he  genetically  several  types,  but  as  yet  evidence  on  this 
point  is  lacking. 

Irregularities  in  the  germ-cells  of  (Enothera  have  been 
observed  by  Gates  of  such  a  kind  that  one  cell  gets  6,  the 


J 


^ 


r 


Fig.  62. — Somatic  chromosomes  groups  of  (Enothera  scintiUans,  showing  variable  numbers 

of  chromosomes.     (After  Hance.) 

other  8  chromosomes.  A  pollen  grain  with  8  chromo- 
somes fertilizing  an  egg  with  7  would  give  a  15-chromo- 
some  type.  When  such  a  15-chromosome  plant  forms  its 
egg-cells  the  supernumerary  chromosome  having  no  mate 
may  go  to  either  pole  of  the  spindle,  hence  eggs  of  two 


VARIATION  OF  CHROMOSOMES  157 

sorts  would  result,  vi^.,  7-  and  8-chromosome  cells.«  Such 
a  plant  if  crossed  to  a  normal  plant  should  give  half  nor- 
mal (14),  half  15-chromosome  types.  Such  plants  have 
been  shown,  in  fact,  to  be  produced  (Lutz).  Other  com- 
binations that  would  give  22,  23,  27,  29  chromosomes  have 
been  reported. 

A  variation  in  the  number  of  the  chromosomes  of  a 
somewhat  different  kind  has  been  described  by  Hance  for 
CEnothera  scmtillans,  one  of  the  15-chromosome  types  of 
0,  Lamarchiana.  No  variation  in  number  was  found  in 
the  germ-tract  of  the  same  individuals  that  consistently 
gave  two  types  of  pollen  grains,  one  with  7  and  the  other 
with  8  chromosomes.  The  number  of  chromosomes  in  the 
somatic  cells  was  found  to  vary  from  15  to  21.  Some  of 
the  groups  are  shown  in  Fig.  62.  When  the  15  chromo- 
somes of  the  type-group  are  measured,  it  is  found  that 
they  can  be  arranged  in  respect  to  length  in  7  pairs,  with 
one  odd  one  (marked  a  in  the  figures).  There  is  also 
found  a  constant  length  difference  between  the  pairs.  In 
those  cases  where  there  are  more  than  15  chromosomes  in 
a  cell,  measurements  show  that  the  pieces  can  be  assigned 
to  particular  chromosomes.  When  this  is  done.  Fig.  63,  the 
lengths  of  the  chromosomes  come  out  as  in  the  typical 
cells.  There  can  be  no  doubt  that  the  extra  chromosomes 
in  these  cases  represent  pieces  that  have  broken  off  from 
typical  chromosomes.  This  process  of  fragmentation 
does  not  destroy  the  ^'individuality  of  the  chromosomes'' 
since  the  increase  in  this  way  of  the  number  of  chromo- 
somes would  not  lead  to  any  immediate  change  in  the 
number  of  the  genes.  The  peculiarity  of  the  mutant  0. 
scintilluns  is  not  connected  with  the  increase  in  the  number 
of  its  chromosome  bodies,  but  rather  to  the  presence  of  a 
15th  chromosome. 

Bridges  has  called  attention  to  a  peculiar  case  in 
Drosophila   (1917)   in  which  an  individual  behaves  as 


'  No  pollen  is  produced  by  most  of  the  lata  plants. 


158 


PHYSICAL  BASIS  OF  HEEEDITY 


though  a  piece  of  the  X-chromosome  (recognizable  from 
its  genes  that  normally  lie  in  the  middle  of  the  chromo- 
some) had  become  attached  to  one  end  of  the  other  X-chro- 
mosome. Owing  to  this  piece  (including  the  region  that 
contains  the  normal  allelomorphs  of  vermilion  and  sable) 
the  individuals  give  unexpected  results  in  relation  to  domi- 
nance or  recessiveness  of  certain  factors.    For  example, 


ABCDEFGHIJKLMNO 


A^BCDEFQHI      JKLMNO 


m 

1 

h 

0 

ABCOErdHijKlMNO 

a 


P 


AeCDE^eVil     JKtMfiO 


b     c 


e     f 


S     K 


k     4 


< 
p 

g 

1 

g                         1 

m 

••    "^ 

:V| 


BCDEFGHt  JKLMNO 


■1 


*    ^ 


9 


k      I 


^  6 

Fig.  63. — Scheme  showing  the  probable  relation  between  the  extra  chromosome  pieces  of 
Fig.  62,  and  the  normal  15  chromosomes  of  this  mutant.     (After  Hanse.) 

a  male  that  contains  the  recessive  genes  for  vermilion  and 
for  sable,  normally  located,  and  having  attached  to  this 
chromosome  the  duplicated  piece  (containing  the  normal 
allelomorphs  of  vermilion  and  sable)  is  in  appearance  a 
wild-type  fly,  instead  of  being  vermilion  sable  as  it  would 
otherwise  be  without  the  piece.  On  the  other  hand,  a 
female  having  one  such  chromosome  and  a  normal  ver- 
milion sable  chromosome  is  in  appearance  not  wild  type 


VARIATION  OF  CHROMOSOMES  159 

(as  might  have  been  expected),  but  shows  vermilion  and 
sable,  because  in  this  case  the  two  recessive  genes  for 
vermilion  and  for  sable  dominate  the  single  normal  allelo- 
morphs. But  a  female  having  two  such  duplicated  chro- 
mosomes {i.e.,  tetraploid  for  the  genes  of  certain  regions 
of  the  sex-chromosome)  is  now  wild  type  in  appearance, 
because  the  two  dominants  dominate  the  two  recessives. 
Such  a  female  crossed  to  a  vermilion  sable  male  gives  wild- 
type  sons  and  vermilion  sable  daughters,  which  is  criss- 
cross inheritance  in  an  opposite  sense  from  that  ordinarily 
met  with  in  Drosophila. 

A  second  instance  discovered  by  Bridges,  but  not  yet 
reported,  seems  best  explained  on  the  assumption  that  a 
piece  taken  from  the  second  chromosome  has  become 
attached  to  the  middle  of  the  third  chromosome.  This 
condition  makes  possible  the  linkage  of  mutant  characters 
to  genes  in  both  the  second  and  the  third  chromosome  at 
the  same  time.  The  second  chromosome  that  lost  a  piece, 
and  the  third  chromosome  that  gained  the  piece  (both  were 
of  course  in  the  same  cell),  have  been  easily  kept  together 
in  the  same  stock  ever  since,  because  in  those  cases  where 
they  become  separated  through  assortment  every  zygote 
that  receives  the  deficient  (2nd)  chromosome  dies  unless 
the  same  zygote  has  received  the  third  chromosome  with 
the  duplicate  piece. 

The  preceding  results  show  that  chromosomes  may 
not  only  gain  genes  by  the  attachment  of  pieces 
(duplication),  but  also  that  chronaosomes  may  lose 
pieces   ( deficiency) . 

Other  instances  of  deficiency  have  been  reported  by 
Bridges  which  can  be  explained  either  as  total  losses  of 
certain  regions,  or  due  to  their  inactivation.  Unless  the 
lost  pieces  happen  to  have  been  retained  as  in  the^  last 
case,  the  distinction  between  these  possibilities  is  difficult. 
A  study  of  one  case  has  shown  that  no  crossing  over  takes 
place  in  the  region  of  deficiency,  although  the  rest  of  the 
chromosome  was  little  or  not  at  all  affected.    As  a  result 


160 


PHYSICAL  BASIS  OF  HEREDITY 


the  chromosome  is  '  *  shortened '  ^  by  an  amount  correspond 
ing  to  the  "length"  of  the  deficient  region. 

It  is  not  Avithout  interest  to  notice  that  in  the  first 
case  the  duplicating  piece  is  attached  to  that  end  of  the 
first  chromosome  where  the  spindle  fibre  is  attached.  In 
the  other  case  the  duplicating  piece  is  attached  to  the 


a 


b 


Fio.  64.— 


An  egg  of  Ascariabivalena  fertilized  by  sperm  of  A.  univalens,  a;   later  stage 

of  same,  b. 


middle  of  the  third  chromosome,  and  in  this  chromosome 
the  spindle  fibre  is  attached  to  the  middle. 

An  interesting  case  of  triploidy  has  been  reported  in 
the  threadworm  Ascarw  (Boveri).  Two  varieties  occur, 
one  with  four  chromosomes  (haploid  two),  and  one  with 
two  (haploid  one).     Rarely  a  female  of  one  variety  is 


a 


*<ff 


a 


Fio.  65. — Diploid  and  haploid  groups  of  the  sundew  Droaera.     (After  Rosenberg.) 

found  that  has  mated  with  a  male  of  the  other  variety. 
The  fertilized  eggs  have  each  three  chromosomes  (Fig. 
64).  As  yet  no  triploid  adults  have  been  met  with,  so  that 
the  method  of  conjugation  of  the  chromosomes  in  the 
triploid  types  is  not  known. 

Rosenberg  crossed  two  species  of  sundew,  Drosera 
longifolia,  with  40  chromosomes  (haploid  20),  and  D. 
rohindifolia,  with  20  chromosomes  (haploid  10),  Fig.  65. 


VARIATION  OP  CHROMOSOMES 


161 


The  hybrid  had  30  chromosomes  (20+10).  He  fomid  that 
when  this  hybrid  produces  its  germ-cells  they  show,  after 
reduction,  20  chromosomes,  which  he  interprets  as  due  to 
10  of  the  rotundifolia  conjugating  with  10  of  the  longi- 
folia.  This  leaves  10  without  mates.  At  the  following 
maturation  division  Rosenberg  describes  the  10  paired 
chromosomes  as  reducing,  sending  one  member  of  each 
dyad  to  one  pole,  the  other  member  to  the  other ;  but  the 


Egj. 


Jperm, 


Zygote. 


amete 


n 


m 


Fig.  66. — A  scheme  illustrating  the  fertilization  of  the  egg  of  one  species  of  moth  by 
the  sperm  of  another,  with  reduction  in  I,  with  no  reduction  in  II,  and  with  partial  reduc- 
tion in  III. 

10  unpaired  chromosomes  are  irregularly  distributed  at 
this  division.  If  the  account  is  confirmed,  the  situation 
is  peculiar,  for  if  the  20  (haploid)  chromosomes  of  longi- 
folia  correspond  to  the  10  (haploid)  of  rotundifolia  it  is 
not  obvious  why  all  20  might  not  find  a  place  alongside 
of  the  10,  unless  chance  or  some  difference  of  length,  etc., 
makes  this  impossible.  This  assumes,  however,  that  longi- 
folia  is  not  tetraploid — if  it  is,  then  a  further  question 
arises  as  to  which  chromosomes  of  each  set  of  three  would 
be  the  ones  most  likely  to  conjugate,  etc. 

Crosses  between  three  species  of  the  moth  Pygcera, 
11 


162 


PHYSICAL  BASIS  OF  HEREDITY 


having  different  chromosomes,  were  made  by  Federley. 
The  hybrids  showed  intermixed  characters  of  both 
parents,  and  their  chromosome  nmnber  was  the  sum  of 
the  haploid  numbers  of  their  parents  (Fig.  66). 

No  reduction  in  number  of  the  chromosomes  takes 
place  in  the  hybrid  at  the  synaptic  stage  (except  perhaps 
for  one  or  two  small  ones),  so  that  the  1st  spermatocytes 
contain  nearly  the  sum  of  the  haploid  number  of  the 


-S$- 


)perm 


Zygote.  Conjugation,        Reduction.     Gamete 


IV 
F.XP 


Fig.  67. — Scheme  illustrating  the  history  of  the  chromosomes,  and  the  back-cross  between  a 
hybrid  male  and  one  or  the  other  parent;  also  between  two  such  hybrid  Fi  individuals. 

parents  {A  and  B)  after  division  of  each  chromosome 
(Fig.  67).  A  second  maturation  division  follows  in  which 
each  chromosome  again  divides,  iis  a  result  each  sperm 
contains  the  full  number  of  chromosomes,  half  paternal, 
half  maternal  {A  and  B).  The  hybrid  female  is  sterile, 
but  the  male  is  fertile.  If  he  is  back-crossed  to  a  female 
of  the  A  race  his  sperm,  carrying  both  sets  of  chromo- 
somes, will  produce  a  3N  individual,  A  -{-  B  -\-  A.  It  will 
have  two  sets  of  the  A  genes  to  one  set  of  B.  In  appear- 
ance the  moth  is  practically  the  same  as  the  F^  hybrid, 
because    both    contain   both    sets    of   chromosomes — the 


VARIATION  OF  CHROMOSOM.ES  163 

double  set  A  A  with  B  not  producing  any  striking  differ- 
ence from  the  single  set  A-\-B.  When  this  second  hybrid 
(3iV)  matures  its  germ-cells,  the  two  homologous  series 
{A  +  A)  mate  with  each  other,  and  then  segregate  at  the 
first  division,  while  the  unmated  i5-series  simply  divides. 
At  the  second  division  both  the  A-  and  the  L'-series  divide, 
thus  giving  to  each  sperm  a  haploid  set  of  chromosomes 
{A  -\-  B).  The  sperm  then  is  the  same  as  the  sperm  of 
the  first  hybrid.  So  long  as  the  back-crossing  continues 
the  outcome  is  expected  to  be  the  same. 

If,  instead  of  back-crossing  the  first  hybrid  to  parent 
Ay  it  is  back-crossed  to  parent  B,  the  same  result  as 
before  takes  place,  except  that  the  second  hybrid  is  now 
A-\-B-]-B.  When  it  matures  its  germ-cells,  the  B's 
unite  and  then  separate,  giving  AB  sperm  as  before. 

Here  then  we  find  a  kind  of  inheritance  that  super- 
ficially appears  to  contradict  the  generality  of  Mendel's 
law  of  segregation.  On  the  contrary,  a  knowledge  of  the 
chromosomal  behavior  shows  that  the  results  are  different 
because  the  mechanism  of  conjugation  of  the  chromosomes 
is  changed,  and  changed  moreover  in  such  a  way  that  on 
the  chromosome  theory  itself  the  results  are  what  are  to 
be  expected. 

These  crosses  are  so  important  that  some  further 
details  may  be  added.  The  whole  (2iV)  and  half  (lA') 
number  of  chromosomes  of  the  three  species  studied  by 
Federley  are  as  follows : 

Whole  Half 

Pygaera  anachoreta 60  30 

Pygaera  curtula 58  ^0 

Pygaera  pigra 46  23 

In  the  hybrid  between  the  first  two  species  the  number  of 
spermatocyte  chromosomes  was  found  to  be  59  (30  -f  29). 
No  union  between  any  of  the  maternal  and  paternal  chro- 
mosomes could  have  taken  place.  But  in  the  hybrid 
formed  by  the  union  of  the  two  more  nearly  related  spe- 
cies, curtula  and  pigra,  the  number  of  spermatocyte  chro- 


164  PHYSICAL  BASIS  OF  HEEEDITY 

mosomes  was  found  to  be  as  a  rule  somewhat  smaller  than 
the  sum  of  the  parental  haploid  numbers,  indicating  that 
one  or  more  had  conjugated.  To  the  extent  to  which  such 
union,  and  the  consequent  reduction,  takes  place,  the 
characters  of  the  second  hybrid  generation  may  differ 
from  those  of  the  first — at  least  if  the  conjugating  pairs 
have  different  factors  in  them. 

A  similar  behavior  of  the  chromosomes  has  been 
described  by  Doncaster  and  Harrison  for  two  species  of 
moth  of  the  genus  Biston  (Fig.  24).  The  hybrids  were 
sterile,  and  no  further  generations  were  raised. 

Federley  later  made  similar  crosses  "with  three  other 
moths.  A  cross  between  SmerintJius  ocellata  (with  27 
chromosomes  as  the  haploid  number)  and  Dilina  tilice 
(with  29)  he  regards  as  a  cross  between  genera.  A  cross 
between  S.  ocellata  and  ;tS^.  populi  (with  28)  he  regards  as  a 
species  cross.  A  cross  between  8.  ocellata  and  8.  ocellata 
var.  planus  he  regards  as  a  racial,  or  varietal,  cross. 
As  before  the  spermatocytes  of  the  hybrid  have  the  sum 
of  the  two  parental  numbers  of  chromosomes  (or  a  few 
less  at  most) .  In  other  words,  conjugation  of  the  chromo- 
somes does  not  take  place.  The  most  unexpected  result  in 
these  combinations  is  that  the  types  that  are  so  alike  as  to 
be  classified  as  varieties  behave  as  regards  conjugation 
like  the  other  two  combinations.  The  results  suggest  that 
ordinary  conjugation  may  not  be  due  to  the  similarity 
of  the  sets  of  genes  carried  by  the  chromosomes  so  much 
as  to  other  peculiarities  of  the  combination. 


CHAPTER  XIV 

SEX-CHROMOSOMES  AND  SEX-LINKED 

INHERITANCE 

The  discovery  that  the  female  in  certain  species  of 
animals  has  two  X-chromosomes  and  the  male  has  only  one 
X-chromosome,  either  with  a  Z-chromosome  in  addition 
(Stevens)  or  without  the  Y  (Wilson),  established  a  view 
first  suggested  by  McClung  that  the  difference  between 
the  sexes  is  connected  with  the  distribution  of  particular 
chromosomes.  Two  interpretations  of  the  facts  have  been 
proposed:  The  first,  and  most  obvious  one,  was  that  the 
presence  of  two  sex-chromosomes  (XX),  in  connection 
with  the  rest  of  the  cell  complex,  causes  a  female  to 
develop ;  while  only  one  sex-chromosome  (X)  in  connection 
with  the  rest  of  the  cell  causes  a  male  to  develop ;  the  sec- 
ond interpretation  was  that  of  XX  and  X  are  merely 
indices  of  sex,  i.e.,  that  the  sex-chromosomes  follow  sex 
and  do  not  determine  sex. 

It  is  now  possible  to  show  that  sex  follows  the  chromo- 
somes and  not  the  reverse,  because  if  a  ''female  produc- 
ing'* sperm  (X)  fertilizes  an  egg  without  an  X  (as  excep- 
tionally occurs)  an  XO  individual  is  produced  that  is  a 
male,  whereas  if  this  same  sperm  had  fertilized  an  egg 
with  an  X,  giving  an  XX  individual,  a  female  would  be 
the  result.  Conversely  when  a  ''male  producing"  Y- 
sperm  fertilizes  an  egg  with  two  X's  (as  exceptionally 
occurs)  an  individual  is  produced  that  is  a  female,  despite 
the  presence  in  her  of  a  Z-chromosome. 

The  Sex-Cheomosome 
It  will  be  convenient  to  treat  the  XX-X7  type  of  com- 
bination first.     I  shall  follow  the  usual  custom  of  calling 
both  X  and  Y  sex-chromosomes. 

165 


166  PHYSICAL  BASIS  OF  HEREDITY 

At  the  time  when  the  polar  bodies  are  extruded  from 
the  egg,  the  two  X^s  separate,  one  passing  out,  the  other 
remaining  in  the  egg.  Every  egg  is  left  with  one  X 
(Fig.  68). 

In  the  male,  the  X  and  Y  conjugate  and  separate  at  one 
of  the  maturation  divisions,  so  that  each  sperm  contains 
either  an  X-  or  a  Z-chromosome  (Fig.  68).  Fertilization 
of  any  egg  {X)  by  an  X-bearing  sperm  produces  a  female 

9  cr 

XX  XV 


Fig.  68. — Scheme  showing  the  relation   of  the  sex-chromosome  to  sex-determination. 

XX-XY  type. 

(XX).    Fertilization  of  any  egg  (X)  by  a  F-bearing  sperm 
produces  a  male  (XY). 

Since  the  two  kinds  of  spermatozoa  are  produced  in 
equal  numbers,  females  and  males  will  be  equal  in  num- 
ber.   The  mechanism  is  self-perpetuating. 

The  Inheritance  of  Factors  Carried  by  the  Sex- 
Chromosomes  in  the  Drosophila  Type 

Since  the  son  gets  his  one  X-chromosome  from  his 
mother,  and  the  Y  from  his  father,  he  inherits  factors 
carried  by  the  sex-chromosomes  in  a  different  way  from 


SEX-CHROMOSOMES  AND  INHERITANCE  167 

the  way  in  which  he  inherits  the  factors  carried  by 
the  other  chromosomes  (autosomes),  because  X  and  Y 
differ  from  each  other  in  a  way  in  which  no  other  chromo- 
somes differ. 

The  recessive  gene  for  white  eyes  (iv)  in  Brosophlla  is 
carried  by  the  X-chromosome.  It  is  inherited  in  the  fol- 
lowing way  (Fig.  69)  :  When  a  male  with  white  eyes  (w) 
is  mated  to  a  red-eyed  female  {WW),  the  Fj  sons  and 
daughters  have  red  eyes.  When  these  are  bred  to  each 
other,  all  the  daughters  have  red  eyes  (50  per  cent.),  half 
the  sons  have  red  eyes  (25  per  cent.)  and  half  the  sons 
have  white  eyes  (25  per  cent.).  The  ratio,  irrespective 
of  sex,  is  three  red  to  one  white,  but  the  white-eyed  flies 
are  found  only  amongst  the  males.  In  the  diagram  (Fig. 
69),  the  relation  of  these  results  to  the  sex-chromosomes 
is  shown.  The  X-chromosome  that  carries  the  normal 
gene  (wild  type)  which  gives  red  eyes  is  indicated  by 
W.  The  X-chromosome  that  carries  the  gene  for  white 
eyes  is  indicated  by  w.  The  rod  with  a  bent  end  stands 
for  the  Z-chromosome. 

The  F^  daughters  contain  one  of  each  kind  of  X-chro- 
mosome. The  2^1  sons  only  one  kind.  The  recom- 
binations that  give  the  F2  results  are  shown  in  the  middle 
of  the  lower  part  of  the  diagram.  Half  of  the  females 
are  seen  to  be  homozygous  for  the  wild-type  gene  (W). 
They  should  never  transmit  white  eyes,  and  they  do  not. 
The  other  half  of  the  females  are  heterozygous  (W/r),  and 
if  mated  to  a  white-eyed  male  should  give  50  per  cent, 
red-eyed  males  and  females,  and  50  per  cent,  white-eyed 
males  and  females.  This  thev  do.  The  red  F.,  sons  (IF) 
should  never  transmit  white  eyes,  nor  the  white-eyed  sons 
{w)  transmit  red  eyes.  These  relations  are  also  known 
to  hold. 

The  reciprocal  cross  (Fig.  70),  viz,,  a  white-eyed 
female  {wiv)  to  a  red-eyed  male  (IF)  gives  red-eyed 
daughters  {wW)  and  white-eyed  sons  {w).  If  these  F^'s 


168 


PHYSICAL  BASIS  OF  HEREDITY 


Fia.  69. — Cross  between  White-eyed  male  and  a  red-eyed  female  of  the  vinegar  fly. 


SEX-CHEOMOSOMES  AND  INHERITANCE  169 


/^^, 


kJ 


Ta 


u 


u 


:l>':riW^^ 


^--^^s  V^V-' 


Fig.  70.— Cross  between  white-eyed  female  and  a  red-eyed  male  of  the  vinegar  fly. 


170  PHYSICAL  BASIS  OF  HEREDITY 

are  "bred  together,  the  results  are  as  follows :  Half  the 
daii.srhters  have  white  eyes  (wiv),  half  red  eyes  (wW) ; 
half  the  sons  have  Avhite  eyes  {w)j  half  red  eyes  (TF).  It 
will  he  seen  tliat  the  red-eyed  Fg  daughters  are  all  hetero- 
zygous, and  should  give  50  per  cent,  white  and  50  per  cent, 
red  offspring  if  mated  to  white-eyed  males.    This  occurs. 

Similar  illustrations  might  be  given  for  any  of  the 
50  sex-linked  characters  of  Drosophila.  Of  these  the  sex- 
linked  lethals  form  the  most  interesting  cases  and  will  be 
spoken  of  in  another  connection. 

Despite  the  fact  that  the  results  in  one  of  the  two  fore- 
going crosses  gave  a  3 :  1  ratio,  and  in  its  reciprocal  a  1 : 1 
ratio,  the  results  in  both  cases  conform  to  Mendel's  first 
law  of  segregation.  The  peculiarity  of  the  1 : 1  ratio  is 
due  to  the  fact  that  the  P^  red-eyed  male  is  in  a  sense 
heterozygous  for  the  wild-type  eye  color  (since  he  has 
but  one  X-chromosome  that  carries  the  factor  for  red 
eyes).  Since  in  the  second  cross  the  F^  male  gets  no  red- 
producing  X  from  either  parent,  he  is  pure  for  white 
eyes  in  the  sense  that  he  has  an  X  bearing  the  factor  for 
white  eyes  and  a  Y  that  bears  no  factor  making  red. 
Plence  this  F^  cross  is  exactly  like  a  back-cross  of  a  hetero- 
zygous female  to  a  recessive  male,  and  gives  the  same 
numerical  result,  viz.,  1:1. 

Cases  of  sex-linked  inheritance  of  this  kind  are  also 
known  in  man.  Color  blindness  in  man  appears  to  follow 
exactly  the  same  procedure  as  sex-linked  inheritance  in  the 
vinegar  fly — at  least  certain  kinds  of  color  blindness  have 
been  sho^vn  to  do  so.  Haemophilia  also  is  sex-linked,  and 
there  are  four  or  five  other  defects  in  man  that  appear 
to  come  under  this  head.  According  to  several  accounts 
there  is  an  unpaired  sex-chromosome  (or  two  of  them) 
in  man,  which  is  also  called  for  by  the  genetic  evidence 
relating  to  sex -linkage  in  man,  but  since  the  female  number 
of  chromosomes  in  man  is  stated  by  Guyer  to  be  24,  and 
by  von  Winiwarter  to  be  48,  it  is  unsafe  as  yet  to  appeal 


SEX-CHROMOSOMES  AND  INHERITANCE  171 


FiQ.  71. — Cross  between  a  yellow,  white-eyed  female  and  a  wild-typeC'gray"), 

red-eyed  male. 


172  PHYSICAL  BASIS  OF  HEREDITY 

to  this  evidence  as  showing  the  identity  of  the  sex-deter- 
mining mechanism  of  man  and  the  vinegar  fly. 

When  two  or  more  sex-linked  characters  are  involved 
at  the  same  time,  the  situation  is  different  only  in  so  far 
as  crossing  over  may  take  place  in  the  female.  It  will  be 
simpler  to  consider  such  a  cross  and  its  reciprocal  in  the 
reverse  order  from  that  just  given.  If  a  female  with 
yellow  wings  {yy)  and  white  eyes  {wiv)  is  crossed  to  a 
wild-type  male,  ''gray^'  wings  (Z)  and  red  eyes  (TF),  the 
sons  are  yellow  white  and  the  daughters  are  gray  red 
(Fig.  71).  Wlien  these  are  inbred  there  are  four  types 
in  F2  (ignoring  sex),  viz.,  the  two  original  combinations 
yellow  white  and  gray  red,  and  the  two  crossover  com- 
binations yellow  red  and  gray  white.  They  occur  in  the 
following  ratios : 

Yellow  white  Gray  red  Yellow  red  Gray  white 

99  per  cent.  1  per  cent. 

In  this  case  the  F^  male  acts  as  a  double  recessive,  reveal- 
ing the  amount  of  crossing  over  in  the  F^  female.  Since 
neither  his  female-producing  nor  his  male-producing 
sperms  carry  factors  that  cover  up  the  characters  carried 
by  the  four  classes  of  gametes  in  the  F^  female,  aU  four 
classes  of  her  gametes  are  revealed  in  their  numerical 
proportions.  Eeciprocally,  when  a  male  with  yellow 
wings  {y)  and  white  eyes  {w)  is  crossed  to  a  wild-type 
female  (gray  {YY)  red  {WW),  both  sons  and  daugh- 
ters are  gray  red,  because  both  get  the  dominating  genes 
for  these  characters  carried  by  the  X-chromosome 
received  from  the  mother.  If  these  F^^^s  are  inbred  (Fig. 
72),  the  F2  females  are  gray  red,  since  each  contains  an 
X  with  the  two  dominant  genes  derived  from  the  father 
whose  genes  have  remained  completely  linked,  as  there 
is  no  crossing  over  in  the  male.  On  the  other  hand, 
there  are  four  kinds  of  JPg  males :  yellow  white ;  gray  red; 
yellow  red;  gray  white;  because  each  male  shows  the 


SEX-CHEOMOSOMBS  AND  INHERITANCE  173 

character  of  his  single  X-chromosome,  and  there  are  four 
kinds  of  these  chromosomes  in  his  mother  on  account  of 
crossing  over  in  the  female.  The  other  sex-chromosome, 
the  Y,  has  no  dominating  influence. 


Fia.  72. — The  F2  results  from  the  reciprocal  cross  of  that  shown  in  Fig.  71. 


Sex-linked  Inheritance  of  the  Abraxas  Type 
In  certain  moths  and  birds  it  has  been  shown  by  the 
genetic  evidence  that  the  female  is  heterozygous  for  sex- 
linked  factors.    The  cytological  evidence,  as  far  as  it  goes, 
supports  this  evidence,  but  for  birds  the  material  is  so 


174  PHYSICAL  BASIS  OF  HEREDITY 

difficult  to  interpret  that  Guyer's  conclusions  do  not  seem 
to  me  as  yet  to  be  on  as  secure  grounds  as  those  of  Seller 's 
for  moths.  Both  descriptions  give,  however,  the  bases  for 
a  consistent  explanation  of  sex-linked  inheritance  in  this 
type  (WZ-ZZ). 

Since  we  do  not  know  as  yet  whether  the  same  or  dif- 
ferent sex  factors  are  involved  in  the  Drosophila  and  in 
the  Abraxas  types,  it  seems  best  not  to  use  the  same  sym- 

fVZ  zz 


IV z  zz 

Fig.  73. — Scheme  showing  the  relation  of  the  eex-chromoeomeB  of  the  moth  (and  of  the  bird) 

in  Bex-determination.     WZ-ZZ  type. 

bols  in  both  for  the  sex-factors.  If  in  both  types  a  single 
sex-factor  is  concerned,  and  if  it  is  the  same  in  both,  the 
conditions  that  make  for  a  female  in  one  case  and  for  a 
male  in  the  other  must  be  due  to  a  difference  in  the 
rest  of  the  hereditary  complex  that  reverses  the  reaction. 
It  would  appear  simpler  to  assume  that  the  sex-factor  itself 
is  different  in  the  two  cases.  If  there  is  more  than  one 
factor  for  sex,  the  two  types  may  have  some  in  com- 
mon, but  the  theoretical  situation  would  remain  the  same. 
For  our  present  purpose  these  possible  distinctions  are 
of  no  importance. 


SEX-CHEOMOSOMES  AND  INHERITANCE    175 

If  the  sex-chromosome  that  carries  the  sex-linked  genes 
in  birds  and  moths  be  symbolized  by  Z,  and  its  homologiie 
that  occurs  in  the  female  by  W,  the  scheme  for  sex-deter- 
mination is  that  shown  in  Fig.  73 :  The  eggs  of  the  female 
extrude  either  one  or  the  other  sex-chromosome.  If  Z 
stays  in,  and  this  egg  is  fertilized  by  a  sperm  (Z-bearing 


LACTICOLOR  9  OL 


CroSULARlATA  c5  CG 


O    (T)  GERM  CEaS  (g) 


GR0S5ULAR1ATA  9  OG , 


CRQSaJLARlATA  6  LG 


O    ®    GERM  CELLS ,  ®    ® 


o  ©■• 

LACTICOLOR  9  OL 


o  ©• 

GR0SSULARIATA90G 


®     ®  ^®     ® 

CROSSULARlATAd  GL     GR055UU\R1ATA  d  CO 


Fig.  74. — Cross  between  Abraxas  lacticolor  female  and  grosaulariata  male. 

also)  a  male  (ZZ)  is  produced;  if  W  stays  in,  and  the  egg 
is  fertilized  by  a  Z-bearing  sperm,  a  female  (WZ)  is 
produced.  The  way  in  which  sex-linked  characters  are 
transmitted  may  be  illustrated  by  the  inheritance  of  a 
color  difference  in  the  currant  moth  Abraxas.  The  wild 
species  (grossulariata)  has  a  mutational  variety  called 
lacticolor,  that  differs  from  the  former  by  having  less 


176 


PHYSICAL  BASIS  OF  HEKEDITY 


black  pigment  in  the  wings.  When  a  dark  (grossulariata) 
male  is  mated  to  a  light  (lacticolor)  female,  both  sons  and 
daughters  are  dark  (Fig.  74).  If  these  are  inbred  all  the 
F2  sons  are  dark,  half  the  daughters  are  dark,  half  light. 
As  the  diagram  shows,  the  distribution  of  the  Z-ohromo- 
some  furnishes  the  mechanism  by  means  of  which  we  can 


C3^05SULAR1ATA  9  OG 


LACnCOLOR  d  LL 


O    ®  GERM  CELLS  ® 

/  \ 
\ 


LACTICOLOR  9  OL 


\ 


GR05SUL^R1ATA  c5  GL 


O    ©^GERMCEaS^^®    ® 


O  ®'' 

GROSSULARIATA  9  OG 


o  ®         ®  ® 

LACTICOLOR  9  OL    GROSSULARIATA  d  LG 


®  ® 

LACTICOLOR  6  LL 


Fia.  75. — CroBB  between  Abraxas  grossulariata  female  and  lacticolor  male. 


explain,  as  in  Brosophila,  the  process  of  sex-linked  inheri- 
tance in  this  moth. 

The  reciprocal  cross  is  shown  in  the  next  diagram  (Fig. 
75)  in  which  a  dark  (gross.)  female  is  mated  to  a  light 
(lact.)  male.  The  daughters  are  light  like  the  father,  the 
sons  dark  like  the  mother — criss-cross  inheritance.  The 
daughters  get  their  one  Z-chromosome  carrying  the  light 


SEX-CHROMOSOMES  AND  INHERITANCE     177 

factor  from  their  f athe  •,  the  sons  get  in  addition  to  a  light 
Z  from  their  father  a  dark  dominating  Z  from  their 
mother.  When  the  F^'s  are  bred  together  four  classes 
result  in  the  proportion  of  1:1:1:1,  when  sex  is  taken 
into  consideration,  or  in  the  ratio  1 : 1  for  the  color  differ- 
ences alone. 

According  to  Doncaster,  the  male  and  the  female 
Abraxas  have  each  56  chromosomes,  i.e.,  the  female  is  ZW 
rather  than  ZO;  but  as  yet  the  sex-chromosomes  as  such 
have  not  been  identified.  That  sex  is  connected  with  such 
chromosomes  is  not  only  established  by  sex-linked  in- 
heritance, but  is  also  indicated  by  an  aberrant  race  of 
Ahraaxis  found  by  Doncaster.  The  males  of  the  race  had 
the  normal  number  of  chromosomes  (56),  but  the  females 
had  only  55  chromosomes.  Doncaster  found  that  in  these 
females  an  unpaired  chromosome,  presumably  the  Z-chro- 
mosome,  was  more  often  thrown  out  into  the  polar  body 
than  left  in  the  Qg^,  so  that  most  of  the  resulting  eggs  had 
only  27  chromosomes.  Any  Qgg  of  this  kind  fertilized 
by  a  spermatozoon  should  give  a  55-chromosome  indivi- 
dual, i.e.,  a  female.  The  few  eggs  that  retained  the 
unpaired  Z-chromosome,  fertilized  by  a  Z-spermatozoon, 
would  be  expected  to  give  rise  to  the  rare  males,  which 
like  normal  males  have  56  chromosomes.  The  excess  of 
females  is  thus  accounted  for,  and  incidentally  the  results 
show  that  the  TF-chromosome  carries  no  essential  factors 
for  the  life  of  the  individual,  since  females  without  it 
develop  and  look  like  normal  females.  Probably  it  is 
empty  as  is  the  Y  of  Drosophila. 

In  poultry  there  are  several  cases  of  sex-linked  inheri- 
tance that  follow  the  Abraxas  type.  One  of  the  most 
striking  cases  is  the  cross  between  Barred  Pl5anouth  Rock 
and  Langshan.  When  a  barred  male  is  crossed  to  a  black 
female,  the  sons  and  daughters  are  barred  (Fig.  76). 
Barring  is  dominant  to  black.  Two  such  F\  's,  inbred,  give 
all  barred  males ;  half  the  hens  are  barred,  half  are  black. 
It  may  be  said  here  that  the  black  grandmother  transmits 

12 


178 


PHYSICAL  BASIS  OF  HEKEDITY 


lier  black  color  to  only  half  of  her  grandsons.  The  chro- 
mosomal explanation  can  obviously  be  worked  out  on  the 
same  scheme  as  in  Abraxas  (Fig.  77).  But  if  Guyer's 
recent  account  of  spermatogenesis  in  birds  is  correct,  the 
situation  is  different.  Guyer  describes  the  ripening  of 
the  sperm  as  follows :  There  are  18  chromosomes  in  the 
male,  including  two  large  Z^s  (16  +  2).  After  synapsis 
there  are  9  double  chromosomes  in  the  first  spermatocyte, 
all  of  which,  except  ZZ  separate  at  the  first  maturation 
division,  8  going  to  one  pole  and  8  to  the  other.     One 


Px 


Darned  (f 


Blacky 

zV 


F, 


z^z"^ 


zV 

Darredc?      Barred  o 


z°z" 


z^' 


zV     zV 

Darredc?   Darredc?     Darredo      Black:  9 

FiQ.  77. — Scheme  showing  the  transmission  of  the  sex-linked  characters  B  =  barred,  and 

b  =black  in  the  cross  shown  in  Fig.  76. 

daughter  cell  gets  both  Z's  ( 8  +  2 ) .  This  cell  then  divides 
again,  the  Z's  presumably  separating  so  that  two  second 
spermatocytes  are  produced,  each  with  9  chromosomes 
(8+1),  including  the  Z.  These  become  the  functional 
sperm.  The  other  spermatocyte,  the  one  without  a  Z, 
may  divide  again,  but  it,  or  its  products,  degenerate,  and 
never  produce  sperm.  According  to  Guyer,  there  are  17 
chromosomes  in  the  female,  including  one  Z.  Presum- 
ably, then,  after  reduction  half  of  the  eggs*  will  contain  a 
Z  (8  +  1),  the  other  half  will  be  without  it  (8).  The  egg 
that  carries  a  Z  (8  +  1),  fertilized  by  a  sperm  (each  sperm 
carries  a  Z  (8  +  1)),  wiU  make  a  male  with  18  chromo- 


Fig.  76. — Cross  between  Barred  Plymouth  Rock  male  and  Black  Langshan  female. 


I\ 


/•; 


mm€k 


Fig.   78. — Cross  between  Black  Langshan  male  and  Barred  Plymouth  Rock  female. 


SEX-CHROMOSOMES  AND  INHERITANCE    179 

somes,  including  two  Z's,  The  egg  that  lacks  a  Z  (8),  fer- 
tilized by  a  sperm  (8  +  1),  makes  a  female  with  17  chro- 
mosomes, including  one  Z. 

This  scheme  gives  consistent  results  for  sex-linked 
inheritance  in  birds.  Since  the  daughter  gets  her  single 
Z-chromosome  from  her  father,  she  will  show  any  sex- 
linked  characters  carried  by  his  Z-chromosome.  If  the 
father  carries  a  sex-linked  dominant  gene  his  sons  and  his 
daughters  will  be  alike.  It  should  be  noticed  that  while 
Guyer's  scheme  gives  the  same  results  so  far  as  sex-link- 


n 


BlackcJ*  Barred 


Barredc?       Black 


15 


zV  tz\  ^  z V    zV 

Barredd    Black  d        DarredQ     Black o 

Fig.  79. — Scheme  showing  the  transmission  of  the  sex-linked  characters  B  =  barred,  and 

b  =black  in  the  cross  shown  in  Fig.  78. 

age  is  concerned,  as  the  one  described  by  Seller  for  some 
moths,  the  machinery  in  the  male  is  different  in  the  two 
cases,  while  that  in  the  female  is  presumably  the  same.  In 
both  the  female  is  heterozygous  for  Z ;  in  the  moth  the 
male  is  homozygous  (ZZ),  but  in  the  bird  the  two  Z'5 
described  by  Guyer  both  go  to  one  pole  at  one  of  the 
maturation  divisions,  and  reduce  at  the  other — a  proce- 
dure not  known  in  any  other  animal. 

In  the  reciprocal  cross  (Fig.  78)  a  black  cock  is  bred 
to  a  barred  hen.  The  sons  are  barred — like  their  mother — 
the  daughters  are  black — like  their  father,  criss-cross 
inheritance.    When  the  barred  Fj  cock  and  the  black  hen 


180  PHYSICAL  BASIS  OF  HEREDITY 

are  inbred,  there  are  four  Fo  classes  with  sex  taken  into 
account  in  the  proportion  of  1:1:1:1;  or  ignoring  sex, 
1  barred  to  1  black.  The  barred  and  the  black  races 
differ  by  one  factor  difference  (Fig.  79),  viz.,  barred  Z^ 
and  its  nonnal  recessive  allelomorph  Z^.  This  seems  to 
mean  that  the  Barred  Plymouth  Rocks  is  a  black  race 
with  an  additional  dominant  factor  for  barring.  The 
Black  Langshan  is  the  same  black  race  but  without  the 
barring  factor. 

Until  quite  recently  no  cases  of  crossing  over  had  been 
observed  in  forms  having  the  Abraxas  type  of  sex-linked 
inheritance,  for,  except  in  one  or  two  cases  in  poultry, 
only  a  single  pair  of  sex-linked  genes  were  known,  and  two 
at  least  must  be  studied  together  in  order  to  demonstrate 
linkage.  Goodale  has  recently  studied  two  sex-linked 
characters  in  poultry,  and  states  that  crossing  over  occurs 
in  the  male,  but  whether  or  not  in  the  female  is  not  stated. 

Sex-detekmination  and  Natural.  Paethenogenesis 

Variations  in  the  ordinary  sex-determining  mechanism 
account  in  some  cases  for  the  normal  output  of  males  and 
females  produced  by  parthenogenesis,  and  determine  the 
exceptional  sex-ratios  of  such  species.  The  honey  bee 
furnishes  the  best  known  example.  The  queen  comes 
from  a  fertilized  Qgg,  and  has  therefore  the  double  (22V) 
number  of  chromosomes.  Her  eggs  give  off  two  polar 
bodies,  hence  have  the  reduced,  or  single  number  of 
chromosomes.  Any  Qgg  that  is  not  fertilized  develops 
parthenogenetically  into  a  male.  If  there  are  two  X-chro- 
mosomes  in  the  bee,  as  in  some  of  the  otier  insects,  the 
Qgg  is  expected  to  contain  only  one  of  them  after  the 
extrusion  of  the  polar  bodies.  Hence,  if  it  develops  with- 
out doubling  its  chromosomes,  it  should  give  rise  to  a 
male.  That  the  male  has  the  smgle  number  of  chromo- 
somes is  also  borne  out  by  the  evidence  from  a  peculiarity 
of  the  first  spermatocyte  division  in  which  the  cytoplasm 
divides,  but  the  chromosomes  do  not  separate  into  two 


SEX-CHROMOSOMES  AND  INHERITANCE     181 


groups.  Several  stages  in  the  maturation  of  the  sperma- 
tozoon of  the  bee  are  shown  in  Fig.  80.  In  a,  the  spindle 
for  the  first  spermatocyte  division  has  appeared.  A  small 
piece  of  the  cytoplasm  cuts  off,  but  the  chromosomes  do 
not  separate,  and  they  return  again  {h  and  c)  to  a  resting 


a 


d 


e 


FiQ.  80. — First  spermatocyte  divisions   a-c,   and   the  second   sperniatocyte  division  d-\i 

in  the  bee.     (After  Meves.) 

stage.  Another  spindle  forms  (eZ),  and  the  chromosomes 
separate  into  two  groups,  one  of  which  is  pinched  off 
as  a  rudimentary  cell  that  never  becomes  a  spermatozoon. 
Hence  only  one,  and  not  four  spermatozooa  as  in  ordi- 
nary cases,  is  formed  from  each  spermatocyte.  In  the 
hornet  (Fig.  81),  the  spermatogenesis  is  similar  to  that  of 
the  bee  in  that  the  first  division  is  abortive.     It  is  different 


182 


PHYSICAL  BASIS  OF  HEEEDITY 


d 


f 


Fig.  81. — First  spermatocyte  division  a-c,  and  the  second  spermatocyte  division  d-f  in 

the  hornet.      (After  Meves.) 

in   that    the    second   division    produces    two    functional 
sperms,  both  female  producing. 

Since  the  male  comes  from  an  unfertilized  egg^  the 


'/^//j///{ : rr 'rr/    rrr 'f//auY/////l'i 


i*=^5>».^«i« 


St^m    ^fff'f/it  I 


41 


r-r 


iliy^aait.  ijo^yrrfuef.x, 


? 


Fig.  S2. — Life  cycle  of  Phylloxera  carycBcaulis. 


SEX-CHROMOSOMES  AND  INHERITANCE    183 

queen  must  transmit  to  him  all  her  characters,  thus  giving 
rise  to  a  form  of  inheritance  that  has  a  superficial  resem- 
blance to  sex-linked  inheritance.  A  queen  of  a  pure  race, 
bred  to  a  male  of  another  race  with  a  dominant  factor, 
produces  daughters  all  showing  the  dominant  character 
of  the  father,  and  sons  all  showing  the  recessive  character 
of  the  mother.  Since  the  son  gets  his  entire  chromosome- 
complex  from  his  mother,  he  must  necessarily  be  like  her, 
whether  the  character  in  question  is  in  the  sex-chromo- 
some, or  in  some  other  one. 


<t?i;/;:'vvi<^.';.;:iS5 


a 


SSt'^ 


■mA 


m. 


mvm^!ffm^m 


Fig.  83. — Extrusion  of  the  polar  body  from  a  male-producing  egg  with  lagging  chro- 
mosomea  on  the  spindle,  a;  and  extrusion  of  the  polar  body  from  a  female-producing 
egg,  6;  in  Phylloxera. 

In  the  phylloxerans  there  are  two  parthenogenetic 
generations  followed  by  a  sexual  one  ( Fig.  82 ) .  In  the  sec- 
ond parthenogenetic  generation  two  whole  chromosomes 
leave  certain  eggs  (Fig.  83)  passing  into  the  single  polar 
body  which  is  given  off  from  the  egg.  Such  eggs  have  two 
less  sex-chromosomes  and  develop  parthenogenetically 
into  males.  In  other  eggs  of  the  same  generation  all  four 
sex-chromosomes  are  retained  after  the  polar  body  is 
produced.  These  eggs  also  develop  parthenogenetically, 
but  produce  females.  Similar  changes  take  place  no  doubt 
in  the  aphids,  for  the  males  have  been  shown  to  have  one 
less  chromosome  than  the  female,  although  the  loss  of  one 


184 


PHYSICAL  BASIS  OF  HEEEDITY 


chromosome  in  the  polar  body  has  not  yet  been  observed 
in  the  group. 

In  both  phylloxerans  and  aphids  there  are  two  classes 
of  sperm  produced  in  the  males  as  in  other  insects,  one 
with  X,  one  without  it.  The  latter  degenerates,  and  only 
the  X  or  female-producing  sperm  remains  functional.  A 
few  stages  in  the  spermatogenesis  of  the  bearberry  aphid 


a 


Fio.  84. — First  and  second  spermatocyte  division  in  the  bearberry  aphid  with  the  formatioai 

of  one  rudimentary  cell. 

are  shown  in  Fig.  84,  Or-g,  In  h,  the  chromosomes  have 
divided  and  moved  to  opposite  poles  while  the  sex-chromo- 
some is  drawn  out  but  has  not  moved  yet  to  either  pole. 
In  c,  the  sex-chromosome  has  been  drawn  into  the  larger 
of  the  two  cells  that  is  produced.  In  d,  the  division  into  a 
larger  and  a  smaller  cell  is  completed.  In  e,  preparations 
for  another  division  are  taking  place  in  the  larger  cell,  and 
in  /  and  g  this  is  completed.  The  smaller  cell  does  not 
divide,  and  later  degenerates.    The  two  spermatozoa  from 


SEX-CHROMOSOMEiS  AND  INHERITANCE    185 

the  two  larger  cells  each  contain  one  X-chromosome  and 
two  autosomes.  They  correspond  obviously  to  the  female- 
producing  sperm  of  other  insects.  Hence  only  females 
arise  from  fertilized  eggs. 

The  rotifers,  especially  Hydatina  senta,  are  the  only 
animals  in  which  the  transition  from  parthenogenetic  to 
sexual  reproduction  has  so  far  been  gotten  under  con- 
trol by  regulating  the  environment,  and  although  the 
evidence  that  the  environment  causes  part  of  its  effects  by 
influencing  the  chromosomal  mechanism  is  not  yet  estab- 
lished, there  is,  in  my  opinion,  some  indication  that  such 
is  the  case.  The  common  method  of  reproduction  in 
Hydatina  is  as  follows:  A  parthenogenetic  female  (Fig. 
85,  A)  lays  eggs  (D),  each  of  which,  after  giving  off  a 
single  polar  body,  develops  at  once  (i.e.,  without  fertiliza- 
tion) into  a  female  like  the  mother.  The  whole  number 
of  chromosomes  is  retained  in  the  eggs.  Several  or  many 
generations  may  be  produced  in  this  way.  Whitney  has 
shown  that  if  such  females  are  fed  on  a  green  alga, 
Euglena,  daughters  appear  (structurally  like  the  others) 
that  produce  smaller  eggs  (E).  If  these  eggs  develop 
without  fertilization  they  become  males  (C).  Examina- 
tion of  these  small  eggs  show  that  they  give  off  two  polar 
bodies,  and  retain  a  reduced  number  of  chromosomes.  This 
process  is  the  same  by  which  the  male  bee  is  produced. 

If  the  female,  that  produces  the  small  eggs  just 
described  from  which  the  males  develop,  should  have  been 
impregnated  by  a  male  soon  after  she  hatched,  her  eggs 
would  then  grow  larger  and  surround  themselves  with  a 
thick-walled  coat.  They  become  the  winter  or  resting 
eggs.  Each  such  egg^  after  the  sperm  enters,  gives  off  two 
polar  bodies,  reducing  in  this  way  the  number  of  its  chro- 
mosomes. By  the  addition  of  the  sperm  nucleus  the  full 
number  of  chromosomes  is  recovered. 

Whitney  has  recently  shown  that  there  are  two  classes 
of  spermatozoa  produced  by  the  male,  large  and  small ; 


186 


PHYSICAL  BASIS  OF  HEREDITY 


for,  owing  to  the  few  sperms  produced  by  each  male  their 
actual  number  can  be  counted.  There  are  twice  as  many 
large  as  small  spei*matozoa,  if,  as  may  be  the  case,  only 
the  large  ones  contain  chromosomes  and  are  functional, 


Fio.  85, — Hydatina  fevta,  adult  female,  A;  young  female  eoon  after  hatching,  B;  adult 
male,  C;  parthenogenetic  egg,  D;  male-producing  egg,  E;  resting  egg,  F.     (After  Whitney.) 

the  conditions  here  would  appear  to  be  like  those  in  the 
hornet,  provided  there  are  no  chromosomes  in  the  small 
spermatozoa.  This  would  also  explain  why  all  fertilized 
eggs  produce  females. 

So  long  as  the  ordinar.y  parthenogenetic  females  are 
fed  on  the  poor  diet  of  Polytoma,  they  continue  to  produce 


SEX-CHROMOSOMES  AND  INHERITANCE    187 

parthenogenetic  females  like  themselves  (Fig.  86),  and 
this  non-sexual  process  contiimes  indefinitely.  If  on  the 
contrary,  parthenogenetic  females  are  fed  abundantly  on  a 
rich  diet  of  the  green  alga  Euglena,  their  eggs  develop  into 
individuals  which,  if  early  fertilized  as  explained  above, 
become  sexual  females,  i.e.,  they  lay  fertilized  eggs,  but  if 
not  fertilized,  produce  small  eggs  that,  developing  par- 
thenogenetically,  become  males.  In  other  words,  the  same 
female  becomes  either  a  sexual  female,  or  a  female  that 


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FiQ.  86. — Diagram  showing  how  a  continuous  diet  of  Polytoma  (P-P)  through  twenty- 
two  months  yielded  only  female-producing  females,  but  when  the  diet  was  suddenly  changed 
to  Chlamydomonas  (at  C),  male-producing  females  appeared  at  once.      (After  Whitney.; 

gives  birth  to  males.  Some  recent  writers,  misunderstand- 
ing these  relations,  have  tried  to  make  it  appear  that  the 
change  here  is  one  that  is  sex-determining,  using  this 
expression  to  all  appearances  as  it  is  ordinarily  employed 
in  other  cases,  but  in  fact  using  the  term  in  such  a  way 
as  to  obscure  the  one  important  fact  that  the  results  really 
show,  viz.,  that  an  environmental  change  of  a  specific  kind 
produces  a  new  kind  of  female  that  is  either  a  producer 
of  eggs  that  become  males  (after  or  because  two  polar 
bodies  are  extruded),  or  becomes  a  sexual  female,  should 
she  early  meet  a  male. 


188  PHYSICAL  BASIS  OF  HEREDITY 

Sex-detekjviination  and  Artificial  Parthenogenesis 

Many  interesting  questions  concerning  sex-determina- 
tion might  be  studied  were  it  as  easy  for  man,  as  it  appears 
to  be  for  nature,  to  make  eggs  develop  without  fertiliza- 
tion. Only  three  cases  are  known  in  which  eggs  developing 
under  artificially  induced  conditions  have  reached  matur- 
ity. Delage  raised  one  sea  urchin  that  had  been  produced 
artificially  to  maturity,  and  determined  that  it  was  a  male. 
Tennent  has  shown  that  the  male  is  heterozygous  for  the 
sex-chromosomes.  Hence,  if  the  artificially  produced 
urchin  has  the  half  number  of  chromosomes  it  should,  if 
like  the  bee,  be  a  male,  but  if,  as  Herlandt  has  shown,  the 
number  of  chromosomes  may  double  before  development, 
a  female  would  be  expected. 

In  the  frog,  Hertwig,  and  later  his  pupil  Kuschake- 
mtch,  found  that  the  number  of  males  is  increased  up  to 
100  per  cent,  if  the  eggs  are  detained  in  the  uterus  for 
one  to  three  days  before  adding  sperm  to  them.  Hertwig 
has  attempted  to  explain  the  result  as  due  to  a  relative 
change  in  the  size  of  the  nucleus  that  takes  place  in  conse- 
quence of  the  delay,  but  since  the  chromosomes  are  at  this 
time  in  the  metaphase  of  the  second  polar  spindle,  it  is  not 
obvious  how  such  an  enlargement  could  be  brought  about, 
quite  aside  from  the  question  as  to  whether  the  result 
imagined  would  follow  even  after  such  a  change.  I  have 
suggested  that  these  eggs  with  deferred  fertilization  may 
develop  parthenogenetically,  due  either  to  the  egg  nucleus 
alone  giving  rise  to  the  nuclei  of  the  embryo,  or  to  the 
sperm  alone  giving  rise  to  these  nuclei,  in  the  latter  case, 
the  polar  spindle  of  the  egg  having  been  caught  at  the  sur- 
face and  prevented  from  taking  part  in  the  development. 
The  possibility  of  the  nuclei  of  the  frog  arising  in  one 
or  the  other  of  these  ways  is  shown  by  the  work  of  Oscar 
and  Gunther  Hertwig  who  have  found  evidence  that  after 
treatment  with  radium,  the  sperm-nucleus  alone  may  give 
rise  to  the  somatic  nuclei  of  the  embryo.     Packard  also 


SEX-CHROMOSOMES  AND  INHERITANCE     189 

has  shown  that  such  kinds  of  androgenetic  embrj^os 
may  arise  in  the  eggs  of  ChcEtoptencs  treated  with  radium, 
and  by  following  every  stage  in  the  process  he  has 
determined  also  that  the  embryos  have  the  reduced 
number  of  chromosomes. 

Other  work  on  the  egg  of  the  sea-urchin  had  seemed 
to  show  that  while  in  most  cases  the  egg,  that  begins  to 
develop  parthenogenetically,  starts  with,  and  continues 
to  maintain  the  half  number  of  chromosomes,  yet  accord- 
ing to  a  recent  observation  of  Brachet,  a  parthenogenetic 
tadpole,  eighteen  days  old,  that  he  produced,  had  the 
double  number  of  chromosomes.  Whether  it  may  turn  out 
that  when  the  egg  nucleus  gives  rise  to  the  nuclei  of  the 
parthenogenetic  individual  it  may  sometimes  double  its 
number  of  chromosomes  (by  failure  of  the  first  cytoplas- 
mic division,  for  example),  and  that  when  a  sperm  gives 
rise  to  these  nuclei  the  half  number  is  retained,  cannot  be 
stated.  Until  we  have  farther  information  on  these  points 
the  expectation  as  to  what  the  sex  of  parthenogenetically 
produced  frog  individuals  will  be  can  only  be  speculative. 
Loeb  has  raised  seventeen  adult,  or  nearly  adult  male 
frogs  and  three  nearly  adult  female  frogs  from  eggs  devel- 
oping after  Bataillon's  puncture  method  of  inducing  par- 
thenogenesis. One  male  frog  had  more  than  the  half  num- 
ber of  chromosomes  (at  least  20  and  presumably  the 
whole  number,  261).  The  number  of  chromosomes  in  the 
females  was  not  determined. 

Gynandeomorphs  and  Sex 
In  the  group  of  insects  especially,  it  has  long  been 
known  that  individuals  occasionally  appear  that  are  part 
male,  part  female.  In  the  most  striking  cases  the  line 
of  division  runs  down  the  middle  of  the  body,  but  there 
are  also  antero-posterior  gynandromorphs,  and  individ- 
uals with  only  a  quadrant  or  even  a  small  piece  of  the  body 
different  from  the  rest  in  its  sex  character.  Several 
hypotheses  have  been  advanced  to  explain  these  rare  com- 


190  PHYSICAL  BASIS  OF  HEREDITY 

binations  of  the  two  sexes,  and  it  is  probable  that  gynan- 
dromorphs  may  arise  in  more  than  one  way,  but  in  Droso- 
phila  it  can  be  demonstrated  that  the  great  majority  of 
gynandromorphs  result  from  dropping  out  of  one  of  the 
sex-chromosomes  at  some  early  division  of  the  fertilized 
egg.  The  demonstration  is  made  possible  by  using  sex- 
linked  characters  that  are  known  to  be  carried  by  the  sex- 
chromosomes.  For  example :  Yellow  body  color  in  Droso- 
j)lula  is  due  to  a  recessive  gene  carried  by  the  X-chromo- 
some.  Its  allelomorph  (wild  type)  lies  also,  of  course,  in 
the  normal  X-chromosome.  If  yellow  is  crossed  to  wild, 
and  a  bilateral  gynandromorph  should  arise,  it  may  be 
yellow  on  the  male  side  (as  seen  in  the  yellow  wings  and 
yellow  hairs  over  half  the  body)  and  wild  type  on  the 
female  side  (Fig.  87). 

Since  the  male  characters  arise  when  only  one  sex- 
chromosome  is  present,  it  must  be  the  yellow-bearing 
chromosome  in  this  case  that  gives  the  male  side.  Since 
the  female  characters  arise  when  two  X's  are  present, 
both  must  be  present  in  the  female  side,  which  will  here 
be  the  wild  type,  since  the  gene  for  wild  type  domi- 
nates the  yellow-producing  gene.  The  gynandromorph 
must  have  arisen,  therefore,  at  a  very  early  nuclear  divi- 
sion in  the  egg  in  which  one  daughter  X-chromosome  failed 
to  pass  into  one  of  the  daughter  nuclei.  The  diagram 
(Fig.  88)  shows  how  such  a  result  might  be  supposed  to 
have  come  about. 

The  diagram  indicates  that  one  daughter  chromosome 
X'  (bearing  the  gray  gene)  has  failed  to  become  incor- 
porated in  its  proper  nucleus,  which  is  therefore  left  with 
only  one  X.  From  this  nucleus  the  nuclei  of  the  male  half 
are  produced,  while  from  the  XX  nucleus  the  nuclei  of  the 
female  half  arise.  That  both  of  these  nuclei,  the  XX  and 
the  X  nucleus  contain  other  chromosomes  derived  from 
both  parents  has  been  shown  by  making  one  of  the  original 
parents  homozygous  for  some  recognizable   autosomal 


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SEX-CHEOMOSOMBS  AND  INHERITANCE    191 

character.  It,  or  its  normal  allelomorph,  should  therefore 
be  present  in  both  nuclei  if  all  the  chromosomes  of  the 
fertilized  egg  have  divided  normally  except  the  X-chromo- 
somes.  This,  in  fact,  has  been  found  to  be  the  case  (Mor- 
gan, Bridges,  Sturtevant). 

Nearly  all  of  the  many  hybrid  gynandromorjjhs  of 
Drosophila  can  be  explained  as  above.  In  a  few  cases, 
when  the  abdomen  of  the  fly  was  sufficiently  female  to 
make  mating  possible,  it  has  been  found  that  the  eggs  give 
the  results  expected  for  a  female  having  the  sex  linked 
factors  that  entered  the  cross. 


FiQ.  88. — Diagram  showing  elimination  of  X'  at  an  early  cell-division,  so  that  the  nucleus 
to  the  right  gets  X  and  X'  and  that  to  the  left  only  X. 

In  a  few  cases  in  Drosophila  the  explanation  of  chro- 
mosomal dislocation  will  not  cover  the  results.  Some  of 
these  cases  can,  however,  be  accounted  for  by  another 
hypothesis.  Should  an  egg  arise  with  two  nuclei  (there 
are  several  possible  ways  for  this  to  occur),  one  nucleus 
having  one  set  of  factors,  the  other  the  other  set  (the 
parent  being  heterozygous),  then  if  each  nucleus  is  sepa- 
rately fertilized  a  different  combination  of  factors  is  pos- 
sible from  that  possible  on  the  elimination  theory.  A 
gynandromorph,  described  by  Toyama,  appears  to  belong 
to  this  category.  Toyama  found  two  gynandromorphs 
of  the  silkworm  (Fig.  89)  whose  mother  belonged  to  a  race 
with  banded  caterpillars,  and  whose  father  belonged  to  a 


192 


PHYSICAL  BASIS  OF  HEREDITY 


race  with  pale  caterpillars.  One  of  these  was  banded  on  the 
left  side  (which  side  was  also  female)  and  pale  on  the  right 
side  (which  was  also  male).  The  sex  of  the  two  sides  was 
only  apparent  after  the  moth  had  appeared.  The  banded 
character  of  the  worm  is  kno^^^n  to  be  dominant  to  the  pale 
character,  but  neither  is  sex-linked.  The  case  can  be 
explained,  if  as  the  evidence  indicates,  the  mother  was 


<ft^i:> 


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rc  t) 


-M 


'%pri 


striped 


gynandromorph 


plain 


Fig.  89. — Caterpillars  of  the  silkworm  moth.       A  striped  one  to  the  left,  a  plain  one  to 
the  right,  a  hybrid  gynandromorph  in  the  middle. 


heterozygous  for  a  not  sex-linked  character,  banded,  and 
if  she  produced  an  egg  with  two  nuclei  (Fig.  90).  Don- 
caster  has  found  such  eggs  in  Abraxas,  and  has  shown 
that  each  nucleus  extrudes  separately  polar  bodies,  and 
that  each  reduced  egg  nucleus  is  fertilized  by  a  separate 
spermatozoon.  If  as  shown  in  the  next  diagram  one 
reduced  nucleus  has  a  TF-chromosome,  and  a  factor  for 
banded  carried  in  one  of  the  autosomes,  and  the  other 
reduced  nucleus  has  a  Z-chromosome,  and  in  one  of  the 


SEX-CHROMOSOMBS  AND  INHERITANCE     193 

autosomes  a  factor  for  pale,  and  if  a  spermatozoon,  carry- 
ing the  factor  for  pale,  fertilizes  each  nucleus,  the  two 
zygotic  nuclei  will  be  ZW  female  and  banded,  and  ZZ 
male  and  pale.  This  gives  at  least  a  formal  explanation 
of  the  results,  and  helps  us  to  see  how  such  a  rare  event, 
the  appearance  of  two  gynandromorphs  in  the  same  brood, 
happened  to  occur  at  the  same  time ;  because,  as  Doncas- 
ter's  evidence  shows,  a  double  nuclear  condition  may  be 
characteristic  of  the  eggs  of  certain  females. 


Fia.  90. — Diagram  illustrating  how  a  heterozygous  egg  with  two  nuclei  fertilized  by  two 
sperms  might  produce  a  gynandromorph  like  that  shown  in  Fig.  89. 


* '  Intebsexes  ' '  AND  Sex  Genes 

The  quantitative  relation  of  one  X  for  male  and  two 
X's  for  female  that  has  been  found  to  hold  in  many  of  the 
groups  of  animals  might  seem  from  a  purely  a  priori 
point  of  view  capable  of  being  modified  in  such  a  way  that 
an  intermediate  condition  might  be  realized,  but  whether 
such  conditions  should  be  expected  to  give  rise  to  her- 
maphrodites or  to  non-sex- somethings  (intermediates)  — 
or  to  a  mosaic  of  both  sexes,  or  should  rather  be  expected 
to  die  could  scarcely  be  foretold.  There  are  three  cases 
in  which  individuals  called  ^ '  intersexes ' '  have  been  found, 
or  produced;  and  since  their  interpretation  has  led  to  a 
view  that  has  appeared  to  contradict  the  ordinaiy  sex- 
determination  scheme,  these  cases  must  be  briefly  referred 
to  here.    Gold«chmidt  has  studied  very  thoroughly  ' '  inter- 

13 


194  PHYSICAL  BASIS  OF  HEEEDITY 

sexes  ^'  that  arise  when  the  European  and  Japanese  race 
of  gypsy  moths,  Lymantria  dispar  and  L.  japonica,  are 
crossed.  Eiddle  has  described  doves  obtained  by  crossing 
the  white  ring  dove  (Streptopelia  alba)  and  the  Japanese 
turtle  dove  (Turtur  orieiUalis)  that  are  intersexual  in 
their  mating  habits.  Olga  Kuttner  and  Banta  have  found 
that  certain  lines  of  Cladocerans  (Simocephalus)  may 
produce  (parthenogenetically)  '^intersexual  individuals'^ 
in  the  sense  that  an  individual  may  possess  some  of 
the  secondary  sexual  differences  of  one  sex  and  some 
of  the  other. 

Some  of  Goldschmidt's  combinations  between  different 
races  of  gypsy  moth  produce  only  intersexual  females,  i.e., 
individuals  that  are  mostly  female,  but  have  also,  in  spots, 
male  characters.  In  the  most  extreme  cases  they  are 
almost  like  males,  not  only  in  color,  but  even  in  the  partial 
production  of  testes.  Other  racial  combinations  give  male 
intersexes,  i.e.,  individuals  that  are  for  the  most  part 
males,  but  show,  in  spots,  some  of  the  characteristics  of 
the  female.  Goldschmidt  explains  these  results  by  the 
assumption  that  the  sex  factors  have  different  quantita- 
tive values  in  the  different  races.  He  represents  the 
female  by  FFMm,  and  the  male  by  FFMM.  If  the  FF ' '  fac- 
torial set''  is  represented  by  80  units,  and  the  '' present" 
male  factor,  M,  by  60  units,  then  the  above  formula  for  the 
female  becomes  80-60  =  +  20,  and  the  male  formula 
becomes  80- (60  +  60)= -40.  In  the  former,  female  units 
'' dominate,"  in  the  latter,  the  male.  Values  like  these 
can  be  arbitrarily  set  for  all  the  different  races.  For 
instance,  to  the  'Sveak"  European  race  and  the  "strong" 
Japanese  the  following  values  are  assigned : 

Weak  European  Race  Strong  Japanese  Race 

9     FF        Mm  FF        Mm 

80,  60  100,        80 

(J^    FF        MM  FF        MM 

80,       60,60  100,     80,80 


SEX-CHEOMOSOMBS  AND  INHERITANCE    195 

If  a  Japanese  female  is  crossed  to  a  European  male, 
the  Fi  female  and  male  may  be  represented  in  the  fol- 
lowing* formula: 

Fi  9  FF    Mm  Fi  c?  FF    MM 

100,         60  100,  80,      60 

Both  ^^normaP'  female  and  male  offspring  are  expected 
in  equal  numbers.  The  reciprocal  cross  gives  a  different 
result,  vi^.: 

Fi  cf  FF    MM  Fi  c?  FF    MM 

80 -(80+60)  80,  80,      60 

The  Fi  female  is  FF-M==0;  and  is  therefore  repre- 
sented as  intersexual.  It  will  be  observed  that  the  so- 
called  ^^female  factors'^  in  these  formulas  are  supposed 
to  be  inherited  entirely  through  the  mother. 

By  assigning  different  values  to  FF  and  M  in  the  dif- 
ferent races  it  is  possible  to  express  the  results  in  such 
a  way  that  the  sexes  obtained  by  various  crosses  have 
different  minimal  values — those  less  or  more  than  any 
assigned  value  for  a  given  sex  are  interpreted  as  inter- 
sexes. In  the  example  cited,  an  exact  balance  (=0) 
between  the  conflicting  factors  produces  an  individual  that 
is  represented  as  neither  male  nor  female.  It  is  not 
obvious,  however,  why  it  should  be  made  up  of  parts  each 
of  which  is  strictly  comparable  to  the  same  part  in  a  male 
or  a  female. 

While  the  assignment  of  arbitrary  values  to  sex  fac- 
tors is  a  legitimate  procedure,  it  is  not  a  quantitative 
analysis  in  the  ordinary  sense,  since  the  quantities 
are  not  referred  to  some  external  measure,  but  are 
purely  arbitrary. 

How  far  an  erratic  elimination  of  sex-chromosomes 
in  later  stages  of  cell-division  might  account  for  the  result 
cannot  be  stated,  since  there  are  at  present  no  facts  to 
go  upon — the  chromosome  count  in  somatic  cells  of  the 
hybrid  has  not  yet  been  reported,  but  Goldschmidt  thinks 


196  PHYSICAL  BASIS  OF  HEREDITY 

that  the  mode  of  development  of  the  embryo  precludes 
this  interpretation. 

Riddle  obtained  his  intersexual  hybrids  by  causing 
their  mother  to  produce  many  more  eggs  than  she  would 
ordinarily  produce.  This  was  done  by  removing  the  eggs 
from  the  nest  as  soon  as  they  were  laid.  Towards  the  end 
of  a  series  obtained  in  this  way  an  overworked  female 
produced  an  excess  of  males.  Some  of  these  males  Riddle 
regards  as  females  that  have  been  changed  into  males — 
the  completeness  of  the  change  being  shown  in  their  sexual 
behavior  towards  other  males,  etc.  But  there  is  involved 
in  the  cross  a  sex-linked  factor  that  behaves,  as  R.  M. 
Strong  had  already  shown  several  years  ago,  as  do  sex- 
linked  factors  in  other  birds.  It  is  thus  possible  to  identify 
the  chromosomal  make-up  of  Riddle 's  intersexual  hybrids. 
His  own  results  show  that  the  hybrids  have  the  expected 
combination  of  chromosomes  for  males.  It  appears,  there- 
fore, that  whatever  it  may  be  that  affects  their  behavior 
their  sex  is  determined  by  their  possessing  the  ordinary 
chromosome  constitution  for  males. 

Hermaphroditism  and  Sex 

As  has  been  shown,  the  sex-mechanism,  whether  XX- 
XY  or  WZ-ZZ,  gives  rise  to  two  kinds  of  individuals — 
males  and  females.  There  are,  however,  many  groups  and 
species  of  animals  where  both  eggs  and  sperm  are  found 
within  the  same  individuals,  and  in  typical  cases  there  are 
in  such  individuals  special  ducts  that  are  outlets  for  the 
male  germ-cells  and  others  for  the  female  germ-cells.  In 
these  hermaphrodites  '^sex-chromosomes^'  are  not  known 
to  be  present,  or  if  present  as  in  Ascaris  nigrovenosa,  they 
act  as  sex  determinants  only  in  alternate  generations. 

The  usual  interpretation  of  the  determination  of  the 
sex-ceUs  of  hermaphrodites  is  that  their  differentiation 
is  determined  by  the  same  kind  of  specific  influences  that 
determine,  for  example,  that  certain  cells  of  the  primitive 
gut  develop  into  liver  cells,  others  into  lung  cells,  still 


SEX-CHROMOSOMES  AND  INHERITANCE     197 

others  into  pancreas  cells,  etc.  There  is  nothing  inconsist- 
ent in  such  a  view  with  the  theory  that  in  other  cases  a 
different  mechanism  produces  different  kinds  of  germ- 
cells.  Logically,  this  viewpoint  is  consistent,  but  I  can 
sympathize  with  efforts  that  are  continually  being  made  to 
find  an  explanation  that  makes  use  of  the  same  kind  of 
process  in  genetic  segregation  and  in  embryonic  differen- 
tiation. In  fact,  in  1902,  while  still  under  the  influence  of 
the  then  recent  advances  in  the  field  of  experimental  em- 
bryology (developmental  mechanics),  I  suggested  that  one 
might  attempt  to  treat  the  phenomenon  of  segregation 
from  the  same  theoretical  standpoint  {viz.,  the  realization 
of  alternative  states)  as  was  then  appealed  to  for  embry- 
onic differentiation.  It  soon  became  apparent  to  me,  how- 
ever, that  (1)  the  two  kinds  of  results  depended  upon 
entirely  different  situations,  and  therefore  need  not  have  a 
common  explanation ;  (2)  that  the  genetic  evidence  showed 
the  improbability  of  explaining  segregation  and  differ- 
entiation in  the  same  way;  (3)  that  special  tests  that  I 
carried  out  failed  to  support  the  supposition  of  a  common 
explanation;  (4)  that  while  no  detailed  explanation  is 
possible  at  present  for  the  general  phenomena  of  specific 
differentiation,  yet  for  Mendelian  segregation  the  reduc- 
tion division  supplies  all  that  the  results  call  for. 

Sex  Ratios 

The  theory  of  sex-determination  has  been  deduced 
from  the  evidence  of  equality  of  males  and  females  as 
well  as  from  the  cytological  evidence.  It  remains  to 
explain  why  in  some  cases  the  machine  fails  to  give 
equality  of  the  two  sexes ;  why,  for  example,  all  fertilized 
eggs  of  phylloxerans  and  aphids,  or  daphnians,  or  roti- 
fers, or  bees,  are  female ;  why  certain  mutant  races  of  flies 
give  twice  as  many  daughters  as  sons;  why  other  races 
of  flies  produce  nearly  all  sons ;  why  the  sex  ratio  in  man 
is  about  106  males  to  100  females. 

It  is  perhaps  needless  to  point  out  that  if,  in  a  species 


198  PHYSICAL  BASIS  OF  HEREDITY 

in  which  sex  is  determined  by  a  chromosome  mechanism, 
it  were  possible  to  change  the  sex  by  other  agencies  in 
spite  of  the  chromosome  arrangement,  the  latter  relation 
would  be  entirely  thrown  out  of  gear  and  males  would 
transmit  sex-linked  characters  and  sex  itself  like  females, 
and  females  like  males.  As  no  such  cases  have  been 
found,  it  is  futile  to  discuss  such  a  possibility. 

It  has  been  shown  that  only  the  female-producing 
sperm  in  phylloxerans  and  aphids  becomes  functional, 
hence  it  is  obvious  why  all  the  fertilized  eggs  develop 
into  females.  In  daphnians  and  other  Crustacea  it  is  not 
known  whether  one  class  of  spermatozoa  degenerates,  but 
the  results  are  explicable  on  such  a  view.  In  rotifers 
the  production  of  males  only  by  certain  females  is  due  to 
the  eggs  developing  by  parthenogenesis  with  the  haploid 
number  of  chromosomes  and  this  explains  also  the  case  of 
the  bees,  wasps  and  other  hymenoptera.  If  a  queen  bee 
is  unfertilized  or  if  her  supply  of  sperm  gives  out  she 
produces  only  males.  If  she  contains  sperms,  then  any 
egg  that  is  fertilized  produces  a  female,  and  as  Petrunke- 
witch  showed  several  years  ago,  spermatozoa  are  to  be 
found  in  eggs  laid  in  worker  cells — such  eggs  being  known 
to  produce  workers  (9  9).  In  rotifers,  too,  the  presence 
of  a  large  and  a  small  class  of  sperm  suggests  that  only 
the  former  is  functional. 

Certain  females  of  Drosophila  give  a  sex  ratio  of  two 
females  to  one  male.  By  making  such  a  female  hetero- 
zygous as  to  her  X-chromosomes  (each  carrying  different 
factors)  it  can  be  determined  that  the  half  of  the  expected 
sons  that  die  are  the  ones  containing  one  of  these  two 
chromosomes.  It  is  easily  possible  by  means  of  linked 
genes  to  locate  a  factor  in  the  sex-chromosome  (Fig.  91) 
and  to  show  that  whenever  it  goes  to  a  male  the  fly  dies. 
All  the  daughters  survive  because  the  lethal  factor  being 
recessive  does  not  harm  a  female  whose  other_  chromo- 
some comes  from  a  normal  father.  The  scheme  is  shown 
on  the  next  page. 


SEX-CHROMOSOMES  AND  INHERITANOE    199 

As  many  as  20  different  lethals  have  been  found  in 
the  Z-chromosomes  of  Drosophila.  Their  occurrence  in 
these  chromosomes  is  first  noticed  by  the  appearance  of 
such  exceptional  sex  ratios.  Lethal  factors  like  these 
need  not  be  thought  of  as  different  in  kind  from  any 
other  mutant  factors.  They  may  mean  only  that  the 
changes  that  they  cause  are  of  such  a  kind,  structural 
or  physiological,  that  the  affected  individual  cannot 
develop  normally.     Some  of  the  lethals  may  be  fatal  in 


Fig.  91. — Scheme  showing  the  transmission  of  a  lethal  sex-linked  factor  in  an  X-chromoiome 

the  black  one  in  the  diagram. 

the  egg  stages,  others  are  known  to  cause  the  death  of 
the  larvae,  others  probably  act  on  the  pupae,  and  a  few- 
even  allow  an  affected  male  to  occasionally  come  through. 
In  man  and  in  several  other  mammals  there  is  at  birth 
a  slight  excess  of  males  over  females.  Since  male  babies 
die  oftener  than  females,  the  difference  has  been  said  to  be 
an  '^adaptation,''  with  the  implication  that  it  calls  for  no 
further  explanation.  Several  possible  solutions  suggest 
themselves.  The  male-producing  sperm  bearing  the  sex- 
chromosome  may  more  frequently  develop  abnormally 
than  the  female-producing  sperm.  Again,  since  the  sper- 
matozoa must,  by  their  own  activity,  travel  the  entire 


200 


PPIYSICAL  BASIS  OF  HEREDITY 


length  of  the  oviduct  to  reach  the  egg  as  it  enters  the 
tube,  the  greater  size  or  weight  of  the  female-producing 
spemi  may  give  a  slight  advantage  to  the  male-produc- 
ing sperm  in  the  long  trip  up  the  tube.  This  would  lead 
to  an  excess  of  males.  There  are  still  other  possibilities, 
which  if  realized,  would  suffice  to  slightly  change  the  equal- 
ity of  the  output  of  the  machine. 

'    NoN-DISJUNCTION 

Females  of  Drosophila  are  occasionally  found  that 
give  exceptional  breeding  results  which  have  been 
explained  by  Bridges  on  the  view  that  these  females  are 


FEMALE 


MALE 


IXT  FEMALE 


FiQ.  92. — Normal  female  and  male  groups  of  chromosomes  of  the  vinegar  fly,  with  the 

XXY  female  group  below. 

XXY  individuals  (Fig.  92).  It  has  been  shown  by  cyto- 
logical  examination  that  such  females  do  actually  contain 
an  additional  T-ehromosome.  The  four  possible  ways  in 
which  these  three  chromosomes  might  be  expected  to 
behave  at  the  reduction  division  when  the  polar  bodies 


il[ 


SEX-CHROMOSOMES  AND  INHERITANCE    201 

are  given  off  by  the  egg  are  shown  in  the  next  diagram 
(Fig.  93).  One  X  may  go  out  of  the  egg,  and  the  other  X 
and  the  Y  stay  in ;  or  one  X  may  stay  in  the  egg  and  the 
other  X  and  the  Y  go  out.  In  these  hvo  cases,  X  and  X 
may  be  thought  of  as  members  of  a  pair  that  conjugate,  as 
in  the  normal  female,  and  then  separate,  and  chance  alone 
determines  whether  the  Y  stays  in  or  passes  out.  Again 
Y  may  go  out  of  the  egg  and  X  and  X  stay  in ;  or  X  and  X 


POLAR 
BODY 


EGGS 


SPERM 


4 

Fig.  93. — Non-disjunction.  In  the  upper  part  of  the  figure  the  four  possible  modes  of 
elimination  of  the  sex-chromosome  from  XXY  eggs  are  shown;  the  results  of  their  fertili- 
zation by  an  X-bearing  sperm  of  the  male  is  shown  below. 

go  out  and  Y  stay  in.  Here  X  and  Y  may  be  supposed  to 
be  members  of  the  conjugating  pair,  and  the  free  X  goes 
to  the  same  pole  as  the  X  that  conjugated. 

In  the  diagram,  each  of  these  four  types  of  eggs  is 
represented  as  fertilized  by  an  X-bearing  sperm.  In  order 
to  make  the  outcome  more  apparent  the  original  XXF 
female  may  be  supposed  to  have  had  white  eyes  (clear 
X^s)  and  the  male  that  fertilized  her  red  eyes  (here  repre- 
sented by  the  black  X  carrying  the  gene  for  red  eyes). 

Four  classes  of  individuals  are  expected :  (1)  Rod-eyed 
females  (XXZ) ;  (2)  red-eyed  females  (XX) ;  (3)  red-eyed 


202 


PHYSICAL  BASIS  OF  HEREDITY 


females  (XXX)  that  die,  and  (4)  red-eyed  males  (XY). 
The  last  are  exceptional,  since  white-eyed  females  nor- 
mally never  produce  anything  but  white-eyed  sons.  Here 
the  exceptional  male  is  due  to  an  egg  without  an  X,  being 
fertilized  by  a '^female-producing"  (or  X-bearing)  sperm. 
The  three  X  individuals  have  never  been  found,  and 
undoubtedly  die,  presumably  from  too  many  X's.  The 
remaining  red  females  are  of  two  kinds,  one  normal  XX 


FOLAP 

BODY 

ECCS 


SPElBIi 


WHITE  d 

\X/HlTEd 

WHITE   9   (EXCEPTION) 

DIl 

5 

6 

7 

8 

Fig.  94. — Non-disjunction.  In  the  upper  part  of  the  figure  the  four  possible  modes 
of  elimination  of  the  sex-chromosome  from  the  XXY  eggs  are  shown,  ancl  the  results  of 
their  fertilization  by  a  Y-bearing  sperm  of  the  male  is  shown  below. 

and  the  other  (XXF),  which  is  expected  to  repeat  the 
exceptional  behavior  of  her  mother.  In  fact,  this  is  what 
she  does. 

In  the  next  diagram  (Fig.  94)  the  fate  of  the  same  four 
kinds  of  eggs  is  shown  if  they  are  fertilized  by  a  F-bearing 
sperm.  Four  classes  of  individuals  are  expected  (5)  white 
males  (XYY) ;  (6)  white  males  (XY) ;  (7)  white  females 
(XXZ) ;  and  (8)  YY  individuals.  No  individuals  having 
the  last  make-up  have  ever  been  found,  and  there  can 
be  no  doubt  that  an  individual  without  at  least  one  X  dies. 
The  white-eyed  females  are  exceptional,  since  white-eyed 


SEX-CHROMOSOMES  AND  INHERITANCE    203 

mothers  by  red-eyed  fathers  have  normally  only  red-eyed 
daughters.  These  exceptional  white-eyed  females  (XXY) 
must  repeat  the  phenomena  of  non-disjunction,  and  it  has 
been  found  that  they  do  so  invariably.  The  white-eyed 
male  XY  is  normal ;  the  other  male  should  produce  some 
XY  sperm  and  thus  transmit  both  X  and  Y  to  some  of  his 
daughters.  Such  daughters  as  get  both  X  and  Y  from 
the  entering  sperm  should  show  non-disjunction.  This 
has  been  proven  to  occur. 

An  analysis  of  the  data  has  shown  that  two  of  the  four 
types  of  eggs  are  more  common  than  the  other  two.  As 
indicated  in  both  diagrams  the  types  of  eggs  that  result 
after  X  and  X  have  united  occurs  in  92  per  cent,  of  the 
cases,  and  since  in  this  type  the  unmated  Y  has  a  random 
distribution,  the  XY  egg  is  found  in  46  per  cent,  of  cases 
and  the  X  egg  in  46  per  cent.  The  more  uncommon  type 
of  egg  would  be  expected  to  result  if  X  and  Y  united  and 
then  separated  while  the  other  X  had  a  random  distribu- 
tion.^ Eight  per  cent,  of  such  cases  occur,  giving  XX  eggs 
in  4  per  cent.,  and  Y  eggs  in  the  other  4  per  cent,  of  cases. 

These  results  not  only  furnish  very  strong  proof  of 
the  chromosome  theory  of  sex,  but  serve  also  to  show  how 
a  knowledge  of  the  actual  mechanism  involved  leads  to 
the  discovery  of  how  a  change  in  the  mechanism  gives  a 
new  output.  The  conclusion  that  females  behaving  in 
this  way  must  contain  a  Z-chromosome  was  confirmed 
by  the  cytological  demonstration  that  showed  in  them  two 
X^s  and  a  Y. 


^  Since  this  was  written  it  has  been  found  that  after  X Y  ajniapsis  the  free 
X  always  goes  to  the  same  pole  as  the  synapsed  X. 


CHAPTER  XV 
PARTHENOGENESIS  AND  PURE  LINES 

In  so  far  as  partheiiogenetic  reproduction  takes  place 
without  reduction  in  number  of  the  chromosomes,  the 
expectation  for  any  character  is  that  it  will  have  the  same 
frequency  distribution  in  successive  generations,  because 
the  chromosome  group  is  identical  in  each  generation. 
There  are  a  few  cases  where  parthenogenetic  inheritance 
has  been  studied.    The  results  conform  to  expectation. 

The  only  difference  between  a  species  reproducing  by 
diploid  parthenogenesis  and  one  propagating  vegetatively 
is  that  in  the  latter  a  group  of  cells  starts  the  new  genera- 
tion and  in  the  former  only  one  cell,  viz.,  an  egg,  that  no 
longer  undergoes  reduction,  or  needs  to  be  fertilized.  In 
both,  the  chromosome  complex  remains  the  same  as  in  the 
parent.  Strictly  analogous  to  the  two  foregoing  methods 
of  propagation  are  the  cases  of  sexual  reproduction  in  a 
homozygous  group  of  individuals,  composed  of  males  and 
females  or  in  a  group  of  hermaphroditic  forms  that  are 
homozygous.  Successive  generations  are  here  also 
expected  to  have  the  same  frequency  distribution,  whether 
selected  or  not,  because  they  have  the  same  germ-plasm. 
Johannsen's  pure  lines  furnish  an  example  of  the  last 
case,  for,  in  principle,  pure  lines,  parthenogenetic  repro- 
duction, and  vegetative  propagation,  are  concerned  with 
nearly  the  same  situation. 

Johannsen  worked  with  one  of  the  garden  beans 
{Phaseolus  vulgaris)  taking  the  weight  of  the  seeds,  in 
some  cases,  and  measuring  their  sizes  in  other  cases.  It 
is  known  that  this  bean  regularly  fertilizes  itself.  As  a 
consequence  of  self-fertilization  there  is  a  tendency  for 
the  descendants  of  any  form  to  become  in  time  homozy- 
gous, even  when  heterozygous  forms  were  present  at  first. 

204 


PARTHENOGENESIS  AND  PURE  LINES    205 

In  fact,  in  a  few  generations  perpetuated  by  self-fertiliza- 
tion with  chance  elimination  of  individuals,  a  homozygous 
race  will  result.  This  comes  about  as  follows:  Starting 
with  a  heterozygous  hermaphroditic  individual,  some  of 
its  offspring  will,  through  recombination  of  factors, 
become  homozygous,  and  if  self-fertilization  prevails  they 
will  continue  homozygous ;  other  offspring  will  be  hetero- 
zygous. From  the  latter  both  homo-  and  hetero-zygous 
offspring  will  again  be  produced,  the  former  remaining 
such  in  later  generations,  the  latter  continuing  the  process 
of  splitting.  Since  only  a  part  of  each  generation  sur- 
vives, there  is  in  the  long  run  a  better  chance  that  the 
homozygous  individuals  will  be  the  survivors,  because 
those  that  have  become  such  in  each  generation  are  fixed, 
and  those  that  are  not  will  continue  to  produce  some 
homozygotes.  There  will  be  in  consequence  a  steady  proc- 
ess of  recurrence  of  homozygotes  which,  on  chance  alone, 
will  sooner  or  later  win  out. 

The  beans  that  Johannsen  worked  with  had  apparently 
reached  a  homozygous  condition,  and  at  the  start  there 
must  have  been  several  such  lines.  He  studied  nineteen 
of  them.  The  offspring  of  any  one  plant  produced  beans 
that  gave  the  same  frequency  distribution  as  the  beans 
of  the  last  generation.  This  condition  continued  through 
all  successive  generations.  It  is  to  be  noted  that  the  beans 
on  any  one  plant  differ  in  size,  but  any  one  will  give  the 
same  frequency  distribution  as  the  beans  of  the  preceding 
generation.  It  made  no  difference  whether  the  larger 
or  the  smaller  beans  were  chosen  for  planting — they  gave 
the  same  group  in  the  next  generation. 

It  is  interesting  to  compare  this  result  with  what  would 
have  happened  had  the  beans  been  propagating  by  cross- 
fertilization  at  the  time  when  Johannsen  began  his  work 
with  them.  If  this  had  been  their  nonnal  method  of 
reproduction  they  would  probably  have  been  heterozygous 
at  the  start,  and  would  have  given  different  genetic  types 
for  several  generations,  even  if  self -fertilized.    Pure  linos 


206  PHYSICAL  BASIS  OF  HEREDITY 

would  have  appeared  only  after  the  beans  had  become 
homozygous  through  repeated  inbreeding.  But  Johann- 
sen,  starting  with  homozygous  beans,  was  able  to  obtain 
his  extremely  important  results,  because  if  selection  could 
bring  about  any  change  it  would  have  to  be  due  to  a 
change  in  the  genes  themselves.  Here,  by  means  of  a 
crucial  experiment,  he  exposed  an  error  that  had  been 
accepted  by  selectionists  from  1859  to  1903.  It  would  have 
been  difficult,  almost  impossible,  to  give  this  demonstra- 
tion on  an}"  plant  or  animal  in  which  self-fertilization  or 
asexual  reproduction  was  not  the  rule ;  for,  if  the  material 
had  been  heterozygous  either  for  the  main  factors  for  a 
character,  or  for  modifying  factors  for  that  character, 
selection  in  one  or  another  direction  would  be  expected 
through  recombination  of  factors  to  change  the  original 
frequency  distribution.  It  is  true  that  any  stock,  even 
such  as  reproduces  by  males  and  females,  may  be  made 
homozygous  by  inbreeding  brother  and  sister  for  ten  or 
more  generations,  but  even  such  stock  would  have  to  be 
constantly  watched  for  mutation. 

Johannsen  defined  a  pure  line  as  a  race  or  family  of 
individuals  descended  through  an  unbroken  series  of  self- 
fertilizations  from  an  ancestor  homozygous  in  all  its 
genes.  By  making  this  definition  precise  he  made  clear 
the  essential  point  of  his  demonstration.  Now"  that  his 
point  is  made,  it  seems  no  longer  necessary  or  even  desir- 
able, I  think,  to  narrow  the  definition  of  a  pure  line  to 
races  that  self-fertilize,  since  this  is  only  one  form  of 
inbreeding,  resulting  in  the  production  of  homozygous 
individuals.  By  extending  the  definition  of  a  pure  line 
to  all  forms  whose  genes  are  the  same  in  all  individuals 
(whether  the  pairs  are  homozygous  or  not),  the  definition 
covers  all  cases  of  parthenogenesis  that  do  not  undergo 
reduction,  and  all  cases  propagating  by  non-sexual  means, 
for,  in  these  cases  the  same  complex  of  genes  is  present 
in  successive  generations. 

Many  plants  are  propagated  by  offshoots,   stolons, 


PARTHENOGENESIS  AND  PURE  LINES    207 

tubers,  cuttings,  etc.  East  has  studied  the  effect  of  selec- 
tion of  tubers  of  certain  races  of  the  common  potato.  A 
race  was  first  grown  from  a  single  tuber.  By  boring  holes 
into  the  tubers  enough  material  could  be  obtained  for  a 
chemical  test  of  the  amount  of  nitrogen  in  them.  The 
rest  of  each  tuber  could,  if  desired,  be  cut  into  pieces  of 
standard  size  and  planted.  Ten  tubers,  high  in  nitrogen, 
and  ten,  low  in  nitrogen,  were  selected.  The  tubers  of  the 
next  generation  showed  that  there  was  no  relation  found 
between  the  amount  of  nitrogen  in  the  original  tuber  and 
in  those  that  came  from  it.    A  repetition  of  the  experi- 


Fia.  95. — A  wingless  aphid  to  the  left  and  a  winged  to  the  right,  both  belonging  to 
the  same  species.     (After  Webster  and  Phillips.) 

ment  in  another  generation  gave  only  meagre  results 
owing  to  drought.  As  far  as  the  facts  went,  this  genera- 
tion, too,  showed  no  effect  of  selection. 

Most  of  the  protozoa  propagate  by  dividing  into  equal 
or  nearly  equal  parts — i.e.,  by  a  process  of  cell-division. 
Jennings  has  studied  the  effect  of  selection  in  a  culture 
of  Paramecium,  all  members  of  which  had  descended  from 
a  single  individual.  No  change  was  induced.  Later,  how- 
ever, working  on  another  protozoon,  Difflugia  corona, 
Jennings  found  that  selection  brought  about  changes  in 
the  direction  of  selection.  In  this  case,  the  method  of 
division  may  possibly  include  irregular  distribution  of  the 
chromatin  material,  and  the  recent  work  of  Hegner  indi- 
cates that  such  an  interpretation  is  not  improbable.    Pos- 


208 


PHYSICAL  BASIS  OF  HEREDITY 


sibly,  too,  the  irregular  distribution  of  ehromatiii  par- 
ticles (chromidia)  in  the  cytoplasm — aside  from  the 
nuclear  phenomena,  or  in  connection  with  them — may 
make  the  results  similar  in  certain  aspects  to  the  distri- 
bution of  plastids  in  certain  plant  cells. 

Many  species  of  plant  lice — aphids — (Fig.  95,  a) 
propagate  throughout  the  summer  by  parthenogenesis. 
There  is  no  chromosomal  reduction  during  the  develop- 
ment of  the  egg.    Each  egg  gives  off  only  one  polar  body. 


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Fig.  96. — Curve  showing  the  non-efFect  of  selection  for  the  first  twelve  generations 
for  increase  in  body  length,  the  heavy  solid  lines  represent  the  fluctuations  of  the  fraternal 
means;  the  light  solid  line  the  fluctuations  of  the  longest  variant;  the  broken  line  the 
fluctuation  of  the  shortest  variant.      (After  Ewing.) 

each  chromosome  splitting  into  two  daughter  chromo- 
somes, so  that  the  egg  retains  the  whole  number  of  chromo- 
somes. Ewing  has  carried  out  an  extensive  experiment 
with  Aphis  avence,  selecting  individuals  through  a  num- 
ber of  generations  for  the  length  of  the  cornicles  (honey- 
dew  tubes),  for  the  length  of  the  antennas,  and  for  body 
length.  Considering  here  only  the  last,  individuals  were 
selected  for  forty-four  generations  in  a  plus  and  in  a 
minus  direction.  The  graph  for  the  fourty-fourth  to  the 
sixty-third  generation  is  shown  in  Fig.  96.  The  heavy 
solid  line  represents  the  fluctuations  of  the  longest  vari- 


PARTHENOGENESIS  AND  PURE  LINES    209 

ants,  the  broken  line  the  fluctuations  of  the  shortest 
variants.  It  was  found  that  much  of  the  fluctuation 
observed  was  connected  with  temperature.  The  tempera- 
ture was  therefore  kept  constant  at  about  65°  F.  for  the 
next  twenty  generations,  and  as  shown  in  Fig.  97,  the 
fluctuation  in  the  fraternal  line  was  cut  down.  No  in- 
fluence of  the  selection  is  observable  in  the  chart.  This 
evidence,  in  conjunction  with  that  for  other  characters, 
shows  that  no  change  takes  place  in  the  characters  of 


63  C*   65   66   67  6i     69   70  7J      71     73      7'/   75  76     77     78     79      20      ?I  n 


IM 


—Curve  showing  the  effect  of  selection  for  the  second  score  of  generations. 

(See  Fig.  96.) 

the  insect  so  long  as  the  same  group  of  chromosomes 
remains.  It  would  be  difficult  to  find  a  better  example 
than  these  parthenogenetic  insects  to  test  the  claim  that 
selection  can  change  the  germ-plasm,  for  here  the  con- 
ditions are  even  simpler  than  in  unisexual  forms  unless 
they  have  first  been  made  homozygous. 

The  aphids  also  furnish  favorable  material  to  illus- 
trate how  the  environment  may  cause  very  great  changes, 
even  when  the  genetic  complex  remains  the  same.  The 
parthenogenetic  aphids  appear  often  as  winged  individ- 
uals (Fig.  95,  6).  There  is  an  entire  change  in  structure 
involving  practically  every  part  of  the  body.    The  winged 

14 


210  PHYSICAL  BASIS  OF  HEREDITY 

and  wingless  individuals  may  differ  more  strikingly  than 
do  species  of  the  same  genus.  The  winged  forms  arising 
from  the  wingless  produce  wingless  forms  again  in  the 
next  generation  that  may  be  identical  with  those  from 
which  they  came.  It  has  long  been  believed  that  environ- 
mental influences  bring  about  these  transitions  in  aphids, 
but  only  recently  has  critical  evidence  been  obtained.  The 
clearest  evidence  is  that  of  Shinji,  with  the  rose  aphid. 
By  sticking  twigs  of  the  rose  in  sand  and  flooding  the  sand 
with  water  containing  substances  in  solution — a  method 
first  suggested  by  W.  T.  Clarke — the  fluid  being  drawn 
up  into  the  leaves  is  sucked  out  by  the  aphids  on  the  leaves. 
As  the  following  table  shows,  young  aphids  reared  on  the 

Winged  Apterous 

IndividualB.  Individuals 

AgNO,    51  0 

CuSo,    34  1 

HgCl,    31  6 

NiSO*   955  5 

SbCla 41  5 

PbCl, 12  2 

SnCl. 579  8 

ZnClj    49  2 

Mg  salts 840  9 

Sugar   365  160 

Alcohol  2  288 

Alum    3  34 

Acetic  acid 0  67 

Na  salts 2  1029 

Ca  salts 1  433 

K  salts 3  324 

Sr  Saltsi 1  220 

Tannin   1  14 

Urea   5  153 

Water,  distilled   0  394 

Water,  tap  and  creek  17  461 

Peptone    15 

salts  of  the  heavy  metals  as  well  as  on  magnesium  salts 
and  sugar  became  winged,  while  those  reared  on  the  other 
substances  in  this  list  remain  apterous.  Here  we  have 
an  excellent  example  of  how  in  one  environment  a  given 
germ-plasm  produces  one  result,  and  in  another  environ- 
ment a  different  result  without  any  intermediate  forms. 


PARTHENOGENESIS  AND  PURE  LINES    211 

The  change  from  wingless  to  winged  aphids  is  far  greater 
than  most  mutational  changes  that  we  know,  yet  must 
involve  a  different  kind  of  change  because  the  result  is 
reversible,  while  a  mutation,  having  once  taken  place,  is 
relatively  irreversible. 

Summing  up,  it  may  be  said  that  the  evidence  shows 
that  whenever  the  same  chromosomal  complex  containing 
the  same  genes  is  found,  the  measurements  of  any  charac- 
ter in  successive  generations  show  the  same  frequency 
distributions  of  the  measurements,  and  the  form  may  be 
said  in  a  general  sense  to  belong  to  a  pure  line.  The 
evidence  shows  that  whether  the  chromosomal  complex  is 
heterozygous  or  homozygous,  the  results  are  the  same, 
so  far  as  the  pure  line  is  concerned ;  but  it  is  also  obvious 
that  in  most  animals  and  plants,  where  redistribution 
(reduction)  of  the  chromosomes  takes  place  in  each  gen- 
eration, only  forms  already  homozygous  will  give  pure 
lines.  This  was  the  special  feature  of  the  material  that 
Johannsen  worked  with,  but  aside  from  its  practical  value 
in  studying  the  selection  problem,  the  limitation  of  the 
definition  of  pure  lines  to  such  an  exceptional  situation 
leaves  out  of  sight  the  wider  bearing  of  the  evidence. 


CHAPTER  XVI 

THE  EMBKYOLOGICAL  AND  CYTOLOGICAL  EVI- 
DENCE THAT  THE  CHROMOSOMES  ARE  THE 
BEARERS  OF  THE  HEREDITARY  UNITS 

Long  before  the  genetic  evidence  brought  forward  its 
abundant  data  that  are  explicable  on  the  theory  that  the 
chromosomes  carry  the  genes,  embryologists  had  already 
found  other  evidence  that  led  them  to  regard  the  chromo- 
somes as  the  bearers  of  the  hereditary  factors.  Taken 
as  a  whole,  this  evidence  makes  out  a  very  strong  case 
for  the  chromosomes,  but  since  it  did  not  establish  the 
relation  beyond  question,  the  genetic  evidence  was  all 
the  more  welcome. 

The  earliest  evidence,  sometimes  cited  in  favor  of 
chromosomal  inheritance,  was  based  on  the  statements 
that  in  some  cases  at  least,  only  the  head  of  the  spermato- 
zoon enters  the  egg.  Since  it  was  then  thought  that  the 
head  is  composed  almost  entirely  of  the  nucleus,  and  since 
the  child  inherits  equally  (in  the  older  parlance)  from  its 
father  and  from  its  mother,  it  followed  that  the  nucleus 
carries  the  hereditary  elements.  When  later  it  became 
known  that  the  head  of  the  sperm  represents  almost 
exclusively  the  mass  of  condensed  chromatin,  it  was  sup- 
posed that  the  chromosomes,  in  particular,  must  be  that 
part  of  the  nucleus  that  is  the  bearer  of  hereditary  charac- 
ters. Such  a  conclusion  received  indirect  support  from 
the  facts,  then  becoming  kno^\Ti,  that  the  chromosomes 
remain  constant  through  successive  generations  of  cells, 
whereas  the  nuclear  sap  becomes  lost  in  the  gen- 
eral cytoplasm  each  time  that  the  nuclear  wall  is  dis- 
solved. It  was  also  found  that  the  spindle  fibres  disappear 
in  the  resting:  stages,  while  the  nuclear  reticulum  (chro- 
matin) remains. 

212 


BEAEERS  OF  HEREDITARY  UNITS        213 

This  evidence  failed,  however,  in  so  far  as  there  might 
be  present  a  certain  amount  of  nuclear  plasm  in  the  sperm- 
head  that  is  carried  in  with  the  head,  and  if  so,  would  be 
later  mixed  with  the  egg  cytoplasm.  The  discovery  that 
at  the  base  of  the  sperm-head  there  is  present  in  some  eggs 
a  centrosome  that  becomes,  through  division,  the  dynamic 
centre  of  the  next  division,  opened  the  door  to  suspicion 
that  the  sperm  might  bring  in  other  things  than  the  chro- 
mosomes to  influence  development,  and  hence  heredity. 

In  conclusion  then,  while  it  may  be  said  that  the  evi- 
dence that  the  sperm-head  alone  enters  the  egg  may  be 
claimed  as  favorable  for  the  chromosome  view^,  it  cannot 
be  accepted  as  critical  proof,  because  it  is  uncertain 
wdiether  other  things  also  may  not  be  brought  in  besides 
the  chromatin  of  the  sperm. 

Boveri^s  evidence  for  chromosomal  heredity  from  di- 
spermic  sea  urchin  eggs  v/as  ojjen  to  less  objection.  It  w^as 
known  that  when  two  sperms  enter  the  sea  urchin's  egg- 
simultaneously,  the  first  division  of  the  egg  is  into  three 
or  into  four  parts,  because  four  (instead  of  two)  division- 
centres  appear  in  these  dispermic  eggs.  It  was  also  known 
that  these  eggs  rarely  produce  normal  embryos  or  larvae. 
Boveri,  studying  the  mode  of  division  of  the  dispermic 
eggs,  found  that  there  was  an  irregular  distribution  of 
the  chromosomes  to  the  three  or  four  poles  that  appear, 
and  consequently  to  the  three  or  four  resulting  cells  (i^'ig. 
98).  The  abnormal  development  of  the  whole  egg  that 
generally  follow^s  might  be  ascribed  to  the  irregular  dis- 
tribution of  chromosomes  to  different  regions;  for,  quite 
apart  from  the  specific  nature  of  each  chromosome  or 
group  of  chromosomes,  the  activity  of  one  region  being 
quantitatively  different  from  that  of  a  corresponding 
region  in  another  part  of  the  egg  might  be  responsible  for 
the  failure  to  develop  normally.  But  Boveri  went  further 
in  his  analysis.  He  shook  apart  the  three  or  four  blasto- 
meres  coming  from  dispermic  eggs  (by  using  Herbst's 
calcium-free  sea-water  method),  and  compared  the  num- 


214 


PHYSICAL  BASIS  OF  HEEEDITY 


ber  that  developed  into  normal  plutei  with  the  number 
of  plutei  from  one-fourth  normally  fertilized  blastomeres. 
From  the  latter  a  large  proportion  give  rise  to  normal 
embryos,  from  tlie  former  normal  embryos  are  rarer. 
Their  greater  rarity,  Boveri  thought  safe  to  attribute  to 
the  chromosomal  deficiencies  present  in  most  of  such  iso- 


FiQ.  98. — Scheme  showing  dispermic  fertilization  of  the  egg  of  the  sea  urchin  with  the 
subsequent  irregular  distribution  of  the  chromosomes.     (After  Boveri.) 


lated  blastomeres.  He  suggested  that  the  chance  of  a 
blastomere  developing  normally  depends  on  its  having 
at  least  one  full  set  of  chromosomes.  For  these  triploid 
sea  urchin  eggs  with  three  times  18  chromosomes,  the 
chance  of  one  full  set  of  chromosomes  getting  into  each 
blastomere  is,  according  to  Boveri  ^s  calculation,  only  one 
to  10,000.  The  chance  of  getting  at  least  one  chromosome 
of  each  kind  in  one  cell  is  greater.  He  concluded  that  the 
few  embryos  he  obtained  came  from  quadrants  that  had  at 
least  one  haploid  set  of  chromosomes.    There  is,  however, 


BEARERS  OF  HEREDITARY  UNITS        215 

to-day  some  uncertainty  concerning  the  assumption  that 
normal  development  is  to  be  expected  if  in  adflition  to 
one  haploid  set  of  chromosomes  other  chromosomes  arc 
also  present,  because  while  one  set  alone  might  permit 
normal  development,  it  is  by  no  means  certain  that  if 
there  were  one,  two,  or  more  additional  chromosomes,  the 
balance  might  not  be  upset  and  abnormal  development  fol- 
low. On  chance  distribution  alone  the  isohition  of  just  one 
set  and  no  more  would  seem  a  very  remote  possibility, 


I 


Vi/ii 


,t^^^ 


^n^i 


X 


1^' 


a 


Fig.  99. — First  division  of  a  hybrid  egg  showing  the  elimination  of  chromosomes  at  the 
equation  of  the  spindle,  o.  The  reciprocal  cross,  6,  shows  no  such  elimination.  (After  Baltzer). 

but  if  there  is  to  some  degree  a  tendency  for  a  group  of 
daughter  chromosomes  to  move  off  together  as  a  result 
of  their  method  of  division,  there  might  be  a  better  chance 
of  such  a  group  getting  into  one  of  the  three  or  four 
blastomeres  than  by  chance  distribution  alone.  At  pres- 
ent it  is  not  possible  to  make  any  calculation  based  on  such 
an  assumption.  While,  therefore,  Boveri^s  argument  can- 
not be  accepted  as  demonstrative,  yet  it  has  probability 
in  its  favor. 

Baltzer  has  found  a  different  kind  of  evidence  of 
chromosomal  influence.    When  the  eggs  of  one  sea  urchin. 


216  PHYSICAL  BASIS  OF  HEEEDITY 

Strongylocentrohis,  are  fertilized  by  the  sperm  of  another 
sea  urchin,  SpliaerecJdnus,  the  segmentation  nucleus, 
formed  by  the  union  of  the  a^^-  and  sperm-nucleus  shows 
irregularities  in  the  movements  of  the  daughter  chromo- 
somes to  the  poles  of  the  spindle.  While  some  of  the 
chromosomes  after  dividing  pass  normally  to  the  poles, 
others  become  scattered  irregularly  between  the  two  poles 
and  fail  to  become  incorporated  in  the  two-daughter  nuclei 
(Fig.  99,(2).     They  appear  to  become  lost  and  take  no 

^wmmr-' '     -mm^ 

^^ 


///iUV 


^  '  /  ■■  1  \  \  -^  a 


a 


Fig.  100. — Fertilization  of  an  egg  that  had  started  to  develop  parthenogenetically. 
The  belated  sperm  unites  with  one  of  the  daughter  chromosomes  groups  only,  a;  an 
earlier  condition  of  the  same  proeedure.      (After  Herbst.) 

part  in  the  further  development.  Counts  of  the  chromo- 
some plates  in  the  later  divisions  of  the  e^g  give  about 
21  chromosomes,  whereas  36  are  expected  as  the  whole 
number.  It  appears  that  15  chromosomes  are  lost,  and 
presumably  they  belong  to  the  foreign  sperm.  Many  of 
these  eggs  develop  abnormally,  but  those  that  reach  the 
pluteus  stage  show  a  maternal  skeleton  only.  This  seems 
to  mean  that  the  sperm  has  done  no  more  than  start  the 
development.  It  has  contributed  nothing,  or  little,  to  the 
embrj'-o,  and  it  seems  reasonable  to  attribute  this  to  the 


BiEARERS  OF  HEREDITARY  UNITS        217 

loss  of  the  paternal  chromosomes,  especially  in  the  light 
of  the  reciprocal  cross. 

In  this  reciprocal  cross,  the  egg  oif  Splicer ecUnm 
is  fertilized  by  the  sperm  of  Strongylocentrotus.  All 
the  chromosomes  of  the  segnientation  nucleus  divide 
and  pass  regularly  to  the  two  poles  (Fig.  99,  &).  The 
hybrid  embryo  shows  characters  of  both  parental  species. 


"*■-"•--       -. "  "^ 

/'o    "oON 
0 


0 


oo 


VOn  O 


0 


0 


O,^' 


\     O; 

i  o  O; 


Fig.    101. — Larval  sea  urchin  seen  in  side  view.     On  one  side  it  shows  hybrid  characters, 
on  the  other  side  it  is  maternal.     The  sizes  of  the  nuclei  on  these  two  sides,  as  seen  in  the 
figure,  coincide  with  the  view  that  the  hybrid  side  is  diploid  and  the  maternal  side  haploid. 
After  Herbst.) 

The  difference  in  the  two  cases  can  be  safely  attributed 
to  the  observed  differences  in  the  fate  of  the  chromosomes, 
rather  than  to  unrecognized  ditferences  in  other  elements 
brought  in  by  the  sperms. 

Herbst 's  experiments  contribute  further  evidence  in 
favor  of  the  chromosome  interpretation.  He  caused  the 
unfertilized  eggs  of  a  sea  urchin  to  begin  to  develop 
parthenogenetically  by  adding  a  little  acid  to  the  sea 
water.  After  five  minutes  the  eggs  w^ere  removed  to  pure 
sea  water,  and  sperm  of  another  species,  Stronciylocen- 


218  PHYSICAL  BASIS  OF  HEREDITY 

trotiiSj  was  added.  The  sperm  entering  the  egg  after  its 
nucleus  had  started  to  divide,  failed  to  reach  the  egg 
nucleus  until  the  latter  had  divided  (Fig.  100).  The  sperm 
nucleus  then  formed  a  nucleus  of  its  own,  that  passed  into 
one  only  of  the  daughter  cells.  This  cell  got  two  nuclei. 
The  other  cell  had  only  one  of  the  daughter  nuclei.  Such 
half-fertilized  eggs  give  rise  to  larvae  that  are  maternal 
on  one  side,  and  hybrid  on  the  other — or  at  least  larvae 
of  this  kind  are  sometimes  found  in  such  cultures  (Fig. 
101),  and  Herbst  believes  it  is  safe  to  refer  them  to  the 
half -fertilized  eggs.  If  so,  there  can  be  little  doubt  that 
the  hybrid  half  owes  its  peculiarities  to  the  presence  of 
both  sets  of  chromosomes  in  its  cells,  while  the  maternal 
half  owes  its  peculiarities  to  its  single  set  of  maternal 
chromosomes.  This  in  itself,  however,  shows  little  more 
than  do  whole  hybrids  and  whole  parthenogenetic  eggs 
themselves,  for  the  demonstration  that  it  is  the  chromo- 
somes and  not  other  constituents  of  the  sperm-nucleus 
that  make  the  difference  in  the  two  sides  rests  on  the 
unproven  inference  that  if  other  things  than  the  nucleus 
are  involved  they  would  be  distributed  equally  throughout 
the  cytoplasm,  but  produce  no  effects.  There  is  no  reason 
to  suppose  that  they  would  be  so  distributed,  and  no  evi- 
dence that  they  are.  Hence  the  proof  is  not  cogent,  how- 
ever probable  it  may  seem  that  only  the  sperm-nucleus  is 
responsible  for  those  cases  where  there  is  a  difference 
in  the  two  sides. 

On  the  whole,  then,  while  I  am  inclined  to  give  much 
weight  to  this  evidence  from  experimental  embryology  as 
very  favorable  to  the  hypothesis  that  the  chromosomes 
carry  the  hereditary  characters,  it  is  the  genetic  evidence 
that  furnishes  convincing  evidence  in  favor  of  this  view. 


CHAPTER  XVII 
CYTOPLASMIC  INHERITANCE 

In  the  preceding  pages  so  much  emphasis  has  heen 
laid  on  the  chromosomes  as  bearers  of  the  hereditary 
material  that  it  may  apj)ear  that  no  very  important  role 
is  left  to  the  rest  of  the  cell.  Such  an  impression  would  be 
quite  misleading;  for  the  evidence  from  embryology 
appears  to  show  that  the  reactions  by  means  of  which 
the  embryo  develops,  and  many  physiological  processes 
themselves,  reside  at  the  time  in  the  cytoplasm.  Further- 
more, there  is  also  genetic  evidence  to  show  that  certain 
forms  of  inheritance  are  the  outcome  of  self -perpetuating 
bodies  in  the  cytoplasm,  most  of  which  go  under  the  name 
of  plastids.  Recognition  of  plastid  inheritance  carries 
with  it  the  idea  that  if  there  are  such  materials  in  the 
cytoplasm  that  are  self-perpetuating  they  will  have  to 
be  taken  into  account  in  any  complete  theory  of  heredity. 

In  the  case  of  certain  chlorophyll  characters  there  is 
excellent  genetic  evidence  to  show  that  a  peculiar  kind  of 
inheritance  is  due  to  the  mode  of  transmission  of  plastids 
in  the  cytoplasm.  There  is  a  race  of  f our-o  ^clocks  known 
as  Mirabilis  Jalapa  albomaculata,  whose  leaves  are  made 
up  of  patches  of  green  and  white.  Such  leaves  are  said 
to  be  checkered  (Fig.  102,  h).  The  amount  of  green,  or  of 
white,  varies  on  different  leaves,  and  on  such  plants  there 
frequently  appear  leaves  and  entire  branches  that  are 
green  and  others  that  are  white.  The  white  is  due  to  the 
absence  of  green  in  the  chlorophyll  grains.  Some  cells 
have  only  green  chlorophyll  bodies,  and  others  only  white, 
still  others  may  have  the  two  mixed  in  various  amounts. 

Correns  has  shown  that  if  the  flowers  on  a  green 
branch  are  self -fertilized  they  produce  only  green  plants, 
and  these  again  only  green  plants.     Flowers  on  wliite 

1219 


220 


PHYSICAL  BASIS  OF  HEREDITY 


brandies  give  only  white  offspring.  Flowers  on  the  check- 
ered branches  give  some  checkered  plants,  some  white 
plants  and  some  green  plants.  The  proportions  in  which 
these  different  types  arise  varies  according  to  the  amount 
of  green  in  the  branch  from  which  the  self-fed  seed  came. 
Wlien  the  ovary  of  a  flower  on  a  green  branch  is  fertil- 
ized by  pollen  from  a  white  branch,  the  plant  produced 
is  green  like  the  maternal  branch.  If  the  ovary  of  a 
flower  on  a  white  branch  is  fertilized  by  pollen  from  a 


Fig.   102. — Gipen  leaf  and  checkered  leaf  of  four-o't-lock.      (After  Baur.) 

green  branch  the  offspring  is  white  like  the  maternal 
branch.  These  and  other  combinations  show  that  this 
color  inheritance  is  only  through  the  mother.  The  results 
are  explicable  on  the  assumption  that  there  are  normal 
(green)  chlorophyll  bodies  and  abnormal  chlorophylless 
bodies,  both  kinds  propagating  in  the  cytoplasm  by  divi- 
sion, and  that  these  two  kinds  are  transmitted  only  through 
the  egg-cell.  The  green  or  white  color  of  the  leaves  of  a 
given  branch  is  an  index  of  the  kind  of  chlorophyll  body 
that  the  ovaries  will  probably  contain.  At  each  division 
of  the  body-cells  the  chlorophyll  grains  present  in  it  are 
sorted  out  more  or  less  at  random — hence  from  a  cell  that 


CYTOPLASMIC  INHERITANCE  221 

contains  both  kinds,  more  white  granules  than  green  ones 
may  at  times  get  into  a  cell,  and  at  other  times  only  white 
granules  will  get  into  one  daughter  cell,  so  that  a  wliite 
branch  arises. 

In  other  species  of  plants  that  have  white  leaves  and 
branches  and  green  leaves  and  branches,  the  cross  may 
give  a  different  result.  Thus  in  Melandrium  and  Antirrhi- 
num, green  by  white  gives  green  F,  (whicliever  way  the 
cross  is  made),  in  F.  there  are  3  green  to  1  white  plant. 
In  this  case  the  results  can  be  explained  as  due  to  the 
action  of  genes  in  the  chromosome  on  the  production  of 
chlorophyll  in  the  cytoplasm— an  action  of  such  a  kind  that 


Fig.   103. — Pelargouium  that  gave  rise  to  a  white  branch.     (After  Baur.) 

the  granules  do  not  develop  green  color  unless  the  (nor- 
mal) gene  is  present,  in  single  dose  at  least.  In  this  case, 
even  if  the  eggs  only  transmit  plastids,  the  F^  individual 
from  a  white-leaved  mother  by  a  green-leaved  father  is 
green,  because  the  paternal  nucleus  introduces  a  gene 
that  causes  the  green  color  to  develop  in  the  plastids.  It 
is  the  segregation  of  the  genes  in  the  germ-cells  of  the  F^ 
individual  that  leads  to  the  3 : 1  ratio  in  F.,  and  not  the 
distribution  of  the  plastids  as  in  the  preceding  case. 

The  most  peculiar  case  is  that  of  Pelargonium  de- 
scribed by  Baur.  White  leaves  and  branches,  and  green 
leaves  and  branches  occur  on  the  same  plant  (Fig.  103). 
Self-fertilized  seeds  from  each  breed  true  to  color  of 
branch.     White  to  green  gives  a  different  result,  viz., 


222 


PHYSICAL  BASIS  OF  HEREDITY 


mosaic  seedlings  with  patches  of  green  and  white  on  stems 
and  leaves  (Fig.  104).  When  these  seedlings  grow  into 
plants,  the  color  of  the  leaves  will  depend  on  the  color 
of  that  part  of  the  stem  from  which  the  terminal  bud,  and 
lateral  buds  grow  out.  If  a  bud  lies  in  a  green  part  of  the 
stem  the  new  part  will  be  green  (Fig.  104,  a) :  if  the  new 
bud  lies  in  a  wliite  part  of  the  stem  the  new  part  will  be 
white  (Fig.  104,  c) :  and  if  it  lies  in  a  partly  green,  partly 
white  region  the  new  part  will  have  some  white,  some 


Fig.  104. — Diagram  to  show  how  a  sectorial  chimera  may  be  produced.  If  the  ter- 
minal bud  has  come  from  a  region  of  the  seedling  entirely  green,  all  of  the  future  leaves 
will  be  green,  o;  if  from  a  region  without  chlorophyll,  all  the  future  leaves  will  be  white,  c; 
but  if  the  terminal  bud  lies  partly  in  one,  partly  in  the  other  region,  some  white  and  some 
green  leaves  will  arise,  b.     (After  Baur.) 

green  parts  (Fig.  104,  h).  The  only  explanation  that  is 
suggested  by  Baur  is  that  in  this  plant  the  plastids  are 
transmitted  both  by  the  egg  and  by  the  pollen.  The  white 
plant  with  defective  plastids  contributes  part  of  the  plas- 
tids in  the  fertilized  egg,  the  green  plant  with  normal 
plastids  the  other  part.  The  fertilized  egg  contains  there- 
fore both  kinds  of  plastids.  During  division  of  the  egg  and 
embryo,  the  granules  become  irregularly  distributed  in 
the  cells.  Whenever  a  cell  gets  only  defective  granules, 
it  and  its  descendants  are  white,  producing  white  parts : 
when  a  cell  gets  mostly  or  only  green  granules,  it  and  its 
descendants  are  green,  producing  green  parts.     Hence 


CYTOPLASMIC  INHERITANCE  223 

arise  the  checkered  seedlings  from  which  white  or  green 
branches  grow  out. 

The  preceding  facts  and  theories  relating  to  plastid 
inheritance  show  that  if  any  element  outside  the  nucleus 
has  the  power  to  propagate  itself  it  may  be  transmitted 
through  the  egg,  and  even  possibly  through  the  sperm 
(pollen)  also.  There  is  no  contradiction  here  iii  any 
sense  to  Mendelian  inheritance  but  only  an  additional  type 
of  inheritance  that  can  be  studied  by  as  exact  methods 
as  those  used  in  Mendelian  work.  The  chief  difference 
between  chromosomal  and  plastid  inheritance  lies  in  the 
orderly  sequence  of  the  distribution  of  the  genes  in  all 
divisions  by  means  of  the  mitotic  figure,  whereas  the  plas- 
tids  are  supposed  to  be  shuffled  about  at  random  to  the 
daughter  cells  (partly  because  their  division  period  does 
not  correspond  with  that  of  the  cell).  This  haphazard 
distribution  of  the  plastids  at  any  and  all  divisions  is  in 
striking  contrast  to  the  sorting  out  of  the  genes  that  occurs 
only  at  one  specific  cell-division  when  the  germ-cells  pass 
through  the  maturation  stage.  Hence  the  orderliness  of 
Mendelian  inheritance  as  contrasted  with  the  more  irregu- 
lar procedure  in  plastid  inheritance. 

To  embryologists  familiar  with  the  fact  that  differen- 
tiation of  the  egg  is  closely  associated  with  the  cleavage 
pattern,  it  was  a  natural  inference  that  in  the  cytoplasm 
lay  the  inherited  characteristics  that  gave  form  to  the 
embryo,  and  even  to  all  of  its  essential  features.  Little 
room  would  seem  to  be  left  for  the  action  of  the  chromo- 
somes except  to  fill  in  the  details  of  the  characters  already 
outlined  by  cytoplasmic  activity.  This  view  might  l)e  la- 
conically referred  to  as  the  theory  of  the  ^'Embr^^o  in  the 
Rough, '^  or  more  generally  as  the  *' Theory  of  the  Organ- 
ism as  a  Whole.''  Boveri  discussed  some  such  view 
(1903),  and  at  first  considered  it  favorably.  It  has  since 
been  seriously  discussed  by  others.  Boveri  pointed  out 
that  when  a  horse  is  crossed  to  an  ass  it  makes  no  differ- 
ence which  way  the  cross  is  made,  for  both  egg  and  sperm 


224  PHYSICAL  BASIS  OF  HEREDITY 


bring'  in  the  characteristics  that  make  the  organism  first 
a  bilateral  one,  then  a  vertebrate,  then  a  mammal,  and, 
lastly,  a  perissodactyl.  In  all  these  aspects,  both  parents 
agree,  and  beyond  these  limits  hybridizing  is  impossible. 
Whatever  the  germ  develops  into  must  contain  these  com- 
mon characters.  The  important  point  to  determine, 
Boveri  thought,  is  whether  the  species  characteristics  are 
or  are  not  in  the  nucleus.  He  concluded,  after  discussing 
the  pros  and  cons,  that  it  is  doubtful  if  these  preformed 
qualities  of  the  egg-protoplasm  extend  beyond  the  larval 
periods,  but  that  in  general  all  characteristics  that  distin- 
guish the  individual  from  all  others  of  its  species  and 
from  the  characteristics  of  related  species  are  inherited 
through  the  chromosomes.  Later  he  restated  his  con- 
clusion as  follows:  ^^All  essential  characteristics  of  the 
individual  and  of  the  species  are  epigenetic,  and  the  deter- 
mination is  brought  about  through  the  nucleus. ' '  Conklin 
at  one  time  expressed  even  more  sharply  the  idea  that 
group  characteristics  may  be  inherited  in  a  different  way 
from  specific  characters  in  the  following  paragraph: 

We  are  vertebrates  because  our  motbei's  were  vertebrates  ajid  pro- 
duced eggs  of  the  vertebrate  pattern;  but  the  color  of  our  skin  and 
hair  and  eyes,  our  sex,  stature,  and  mental  peculiarities  were  determined 
by  the  sperm  as  well  as  by  the  egg  from  which  we  came.  There  is 
evidence  that  the  chromosomes  of  the  egg  and  sperm  are  the  seat  of 
the  differential  factors  or  determinei's  for  Mendelian  characters,  while 
the  general  polarity,  sjnnmetry  and  pattern  of  the  embryo  are  deter- 
mined by  the  cytoplasm  of  the  egg. 

In  another  statement,  however,  Conklin  takes  what 
seems  to  me  to  be  more  nearly  a  correct  view  in  regard  to 
the  question,  viz.,  that '  ^  There  is  no  doubt  that  most  of  the 
differentiations  of  the  egg  cytoplasm  have  arisen  during 
the  ovarian  history  of  the  egg,  and  as  a  result  of  the 
interaction  of  nucleus  and  cytoplasm ;  but  the  fact  remains 
that  at  the  time  of  fertilization  the  hereditary  potencies 
of  the  two  germ-cells  are  not  equal,  all  the  early  stages  of 
development,  including  the  polarity,  symmetry,  type  of 


CYTOPLASMIC  INHERITANCE  225 

cleavage,  and  the  pattern,  or  relative  positions  and  pro- 
portions of  future  organs,  l)cing-  foreshadowed  in  the  cyto- 
plasm of  the  egg-cell,  while  only  the  differentiations  of 
later  development  are  influenced  by  the  sperm.  In  short, 
the  egg  cytoplasm  fixes  the  general  type  of  development, 
and  the  sperm  and  egg  nuclei  supply  only  the  details." 
If,  as  implied,  the  egg  nucleus  at  first  has  already  pro- 
duced its  effect  on  the  cytoplasm,  it  has  done  something 
more  than  supply  the  details ;  and  as  to  the  sperm  nucleus 
I  should  substitute  nearly  all  the  stages  of  development 
later  than  the  gastrula.  Moreover,  sex  is  certainly  one 
of  the  fundamental  characters  of  the  organism,  yet  it 
appears  to  be  determined  at  fertilization  by  the  chromo- 
somal combination  formed  at  that  time.  Conklin  later 
abandoned  his  earlier  interpretation. 

Quite  recently,  in  his  book  on  ^'The  Organism  as  a 
Whole,"  Loeb  has  discussed  the  question  as  to  whether 
the  protoplasm  of  the  egg  is  ^Hhe  future  embryo  in  the 
rough,  ^ '  the  sperm  furnishing  only  the  ^ '  individual  charac- 
ters. ' '  Loeb  suggests  that  the ' '  specificity  of  the  species ' ' 
must  be  due  to  their  proteins,  and  that  the '  'heredity  of  the 
genus  is  determined  by  proteins  of  a  definite  constitution 
ditfering  from  the  proteins  of  other  genera.  This  consti- 
tution of  the  proteins  would  therefore  be  responsible  for 
the  genus  heredity.  The  different  species  of  a  genus  have 
all  the  same  genus  proteins,  but  the  proteins  of  the  species 
of  the  same  genus  are  apparently  different  again  in  chemi- 
cal constitution  and  hence  may  give  rise  to  the  specific  bio- 
logical or  immunity  reactions. ' '  The  possible  relations  of 
these  considerations  to  heredity  are  summed  up  in  the 
following  paragraph : 

It  is  thu3  doubtful  whether  or  not  any  of  the  constituents  of  the 
nucleus  contribute  to  the  detennination  of  the  species.  This  in  it^ 
ultimate  consequences  might  lead  to  the  idea  that  the  Mendelian  chara<j- 
ters  which  are  equally  transmitted  by  egg  and  spei-matozoou  detenuine 
the  individual  or  variety  heredity,  but  not  the  genus  or  species  hei-edity. 
It  is,  in  our  present  state  of  knowledge,  impossible  to  cause  a  spei-mato- 

15 


226  PHYSICAL  BASIS  OF  HEEEDITY 

zoon  to  develop  into  an  embiyo,  while  we  can  induce  the  egg  to  develop 
into  an  embryo  without  a  spermatozoon.  This  may  mean  that  the 
protoplasm  of  the  egg  is  the  future  embryo,  while  the  chromosomes  of 
both  egg  and  speim  nuclei  furnish  only  the  individual  characters. 

The  evidence  from  Mendelian  heredity  is  adverse  to 
any  such  distinctions  as  those  made  by  the  three  authors 
referred  to  above.  We  find  in  them,  I  think,  an  echo  of  an 
old  and  somewhat  mystical  conception  of  fundamental  dis- 
tinctions between  order,  family  and  generic  characters 
of  animals  and  plants — ^distinctions  that  even  most  syste- 
matic writers  recognize  to-day  as  little  more  than  conven- 
tions that  change  from  group  to  group.  In  the  second 
place,  since  the  cytoplasm  of  the  egg  has  been  under  the 
influence  of  its  own  nucleus  with  a  paternal  and  a  maternal 
group  of  chromosomes  there  is  no  direct  means  of  deter- 
mining whether  its  characteristics  are  due  to  such  an 
influence  or  have  always  been  free  from  it.  The  fact  that 
sperm  of  a  foreign  species  does  not  change  the  cytoplasm 
of  the  egg  at  once  is  to  be  expected  even  from  a  chemical 
viewpoint.  Mendelian  workers  can  find  no  distinction 
in  heredity  between  characteristics  that  might  be  called 
ordinal  or  specific,  or  fundamental,  and  those  called  **  indi- 
vidual.^^ This  failure  can  scarcely  be  attributable  to  a 
desire  to  magnify  the  importance  of  Mendelian  heredity, 
but  rather  to  experience  with  hereditary  characters.  That 
there  may  be  substances  in  the  cytoplasm  that  propagate 
themselves  there  and  that  are  outside  the  influence  of  the 
nucleus,  must,  of  course,  be  at  once  conceded  as  possible 
despite  the  fact  that,  aside  from  certain  plastids,  all  the 
Mendelian  evidence  fails  to  show  that  there  are  such  char- 
acters. In  a  word,  the  distinction  set  up  between  generic 
versus  specific  characters  or  even  *' specificity '*  seems  at 
present  to  lack  any  support  in  fact. 


CHAPTER  XVIII 
MATERNAL  INHERITANCE 

Theke  is  a  kind  of  inheritance  shown  by  eggs  and 
embryos,  sometimes  called  maternal  inheritance,  that  is 
not  the  same  as  plastid  inheritance,  even  although  the  lat- 
ter is  maternal  in  another  sense.  Nor  is  this  so-called 
maternal  inheritance  to  be  confused  with  cases  of  inheri- 
tance in  which  all  or  some  of  the  paternal  chromosomes 
fail  to  function,  leaving  the  embryo  at  the  mercy  of  its 
maternal  set  alone.  Nor  should  it  be  confused  with  sex- 
linked  inheritance  w^here  the  son  gets  certain  characters 
only  from  the  mother,  because  he  gets  his  single  sex- 
chromosome  from  her. 

''True''  maternal  inheritance  relates  to  peculiarities 
of  the  egg  or  larva  that  are  due  to  materials  already  pres- 
ent in  the  egg-cytoplasm  when  the  egg  is  laid.  For  exam- 
ple, if  pigment  is  scattered  in  the  egg,  it  may  collect  in 
certain  regions  after  fertilization,  and  produce  color  in 
them,  as  does  the  yellow  pigment  in  the  egg  of  Cynthia, 
studied  by  Conklin.  In  this  ascidian,  much  of  the  yellow 
pigment  is  carried  at  the  moment  of  fertilization  to  that 
part  of  the  egg  that  later  goes  into  the  muscle  of  the  tail. 
If  the  sperm  used  to  fertilize  such  an  egg  should  come 
from  a  species  without  pigment  in  the  egg,  the  inheritance 
of  color  of  the  young  embryo  would  obviously  be  entirely 
maternal.  In  cases  like  this  one,  the  formed  material, 
or  any  substance  producing  such  materials,  is  already 
present  in  the  cytoplasm,  but  whether  it  has  always  been 
free  from  nuclear  influence  must  be  shown  bv  other  tests. 
In  only  one  cross,  vi^.,  in  the  silkworm,  has  a  third  genera- 
tion been  raised,  and  until  this  has  been  done  in 
others  we  cannot  know  whether  we  are  dealing  in  them 
with  plastid  or  with  deferred  nuclear  influence  (''ma- 
ternal inheritance''). 

227 


228 


PHYSICAL  BASIS  OF  HEEEDITY 


In  certain  races  of  the  domesticated  silkworm  moth, 
Toyama  has  shown  that  pigment  develops  in  the  em- 
bryonic membrane  (serosa)  which,  seen  through  the 
egg-shell,  gives  a  specific  color  character  to  the  embryo. 
It  is  not  clear  from  Toyama 's  account  whether  the  pig- 
ment is  present  at  first,  scattered  in  the  cytoplasm,  and 
collects  later  at  the  surface,  or  whether  it  develops  only 
after  the  embryo  develops.  When  races  are  crossed  with 
characteristic  but  different  embryonic  membranes,  the 
color  of  the  hybrid  is  like  that  of  the  maternal  race  only. 


Pi 


Fl  ^ 


9D  by  cf  rO 


O9H  -by  (f  D< 


ypA 


Eggs  and  erabryo 

Genetic 
constitution. 


Eggs  and  embryo 

Genetic  consti- 
tution of  F2 
individuals . 


DD 


D  R 


LR 


P3     Eggs 


1 


D 
2 


RR 


O 
R 

1 


DD 


1 


o 

D  R 


PR 


0 

D 
2 


RR 


o 

R 

1 


Fig.  105. — Diagram  to  illustrate  maternal  inheritance.     The  black  circle  stands  for  a  dom- 
inant character  affecting  the  serosa  coat  of  the  embryo. 

If  adults  (jPi)  are  raised  from  these  eggs,  it  is  found 
when  they  in  turn  produce  embryos,  that  the  color  of  their 
embryonic  membrane  is  determined  by  the  dominant  char- 
acter of  the  preceding  generation  that  had  been  carried 
in  the  chromosomes,  irrespective  of  w^hether  it  came  in 
from  the  father  or  the  mother  (Fig.  105).  That  the  result 
is  really  chromosomal  is  shown  by  still  another  generation 
in  which  some  of  the  females  show  the  dominant  character 
in  the  membranes  of  their  embryos  and  others  no  color 
in  the  ratio  of  3:1. 

It  appears  therefore  in  this  case,  the  only  one  known 
that  furnishes  critical  evidence,  that  maternal  inheritance 


MATERNAL  INHERITANCE  229 

does  not  differ  in  any  essential  respect  from  ordinary 
Mendelian  heredity. 

A  peculiar  case  that  in  some  respects  and  in  certain 
combinations  appears  to  be  maternal  inheritance,  is 
shown  in  the  character  of  the  seed  of  com  {Zea  nuzis). 

The  endosperm  of  maize  is  produced,  as  in  some  other 
plants,  at  the  time  of  fertilization — one  pollen  nucleus 
unites  with  the  egg  to  form  the  embryo,  another  pollen 
nucleus  unites  with  two  nuclei  in  the  embryo-sac  to  pro- 
duce the  endosperm  whose  cells,  therefore,  are  triploid. 
Floury  corns  have  an  endosperm,  that  is  almost  wholly 
made  up  of  cells  containing ' '  soft"  starch,  while  flint  corns 
have  only  a  small  amount  of  soft  starch  in  the  centre  of 
the  seed  which  is  surrounded  by  a  large  amount  of  hard 
^'corneous"  starch.  Hayes  and  East  have  shown  that  it* 
a  floury  corn  be  used  as  the  mother  and  a  flint  corn  as  the 
father,  the  seeds  are  floury  like  those  of  the  pure  race  of 
floury  corn.  If  a  flint  corn  be  used  as  the  mother  and  a 
floury  corn  as  the  father,  the  seeds  are  flinty.  In  both 
cases  there  is  apparently  maternal  inheritance,  at  least 
as  far  as  the  endosperm  is  involved,  which  is  not,  how- 
ever, a  part  of  the  embryo  proper.  If  the  seeds  from  races 
of  the  foregoing  crosses  are  sown  and  the  plants  allowed 
to  self -fertilize,  the  following  results  are  obtained :  The 
F^  derived  from  floury  2  by  flint  S  produces  both  floury 
and  flint  in  F^  in  the  ratio  of  1 : 1.  The  F^  flinty  reciprocal 
cross  gives  exactly  the  same  result.  The  explanation  of 
the  F^  and  F2  results  is  as  follows :  If  the  factor  for  flinty 
be  F,  and  that  for  floury  be  /,  then  in  the  first  cross  the 
endosperm  is  ffF  and  in  the  reciprocal  cross  FFf.  Since 
ffF  is  floury  and  FFf  flinty  it  follows  that  two  doses  of 
floury  dominate  over  one  dose  of  flinty,  and  conversely 
two  doses  of  flinty  dominate  over  one  dose  of  floury. 

The  F^  embryo,  however,  in  each  of  the  crosses  has 
only  one  F  and  one  /  factor  (F/).  Its  gametes  are  F  and 
/,  and  so  are  its  endosperm  nuclei  which,  as  shown  by 
Weatherwax  have  the  same  reduced  formula  as  the  ovules 


230  PHYSICAL  BASIS  OF  HEEEDITY 

in  the  embryo  sac.  Hence  half  the  embrj^o  sacs  are  F  and 
half  /.  The  former,  F  {-\-F),  fertilized  by  F  pollen  gives 
FFF  endosperm,  by  /  pollen  give  FFf ;  the  latter,  /  ( +/) , 
fertilized  by  F  pollen  gives  ffF  endosperm,  by  /  pollen 
///  endospeim.  The  four  kinds  of  endosperm  fall  into  two 
classes,  soft  and  hard,  in  the  ratio  of  1 : 1  in  the  F^  seeds. 

There  are  races  of  maize  with  yellow  dominant  endo- 
sperm and  others  with  recessive  white.  If  the  mother 
belongs  to  a  yellow  race  and  the  father  to  a  white  one, 
the  F^  endosperm  is  yellow  like  the  mother.  In  the  recip- 
rocal cross  it  is  also  yellow.  If,  however,  races  with  floury 
seeds  are  used,  the  F^  yellow  endosperm  in  the  former 
cross  is  somewhat  paler  than  the  pure  yellow  of  the  yellow 
race.  Eaces  with  purple  or  with  red  endosperm  crossed  to 
white  give  the  same  results,  except  that  in  these  crosses  the 
quantitative  effects  seen  in  the  floury  flint  crosses  are  not 
observed,  for,  one  dose  of  the  dominant  (purple)  to  two 
doses  of  white  gives  the  same  color  as  two  doses  of  purple 
to  one  dose  of  white. 

There  are  two  kinds  of  maize  with  white  endosperm. 
These  when  crossed  together  give  F^  colored  endosperm. 
In  these  cases  one  race  has  one  of  the  factors  for  color,  and 
the  other,  another  complementary  factor — like  the  two 
vv'hite  sweet  peas.  There  is  also  a  race  with  a  dominant 
white  endosperm  factor.  The  occurrence  of  these  kinds  of 
whites  led  to  some  confusion  in  the  earlier  experiments  of 
Correns  on  endosperm  inheritance.  The  word  Xenia,  that 
had  earlier  a  different  meaning,  is  used  to-day  for  these 
cases  of  double  fertilization  in  which  the  pollen  has  an 
influence  on  the  seed  (the  endosperm)  that  is  not  a  part  of 
the  Fj  plant  itself.  East  and  Hayes  sum  up  the  results 
given  above  (exclusive  of  the  floury -flint  cross)  as  follows : 

When  two  races  differ  in  a  single  visible  endosperm  character  in 
which  dominance  is  complete,  Xenia  occurs  only  when  the  dominant 
parent  is  the  male;  when  they  differ  in  a  single  visible  endosperm 
character  in  which  dominance  is  incomplete,  or  in  two  characters  both 
of  which  are  necessary  for  the  development  of  the  visible  difference, 
Xenia  occurs  when  either  is  the  male. 


MATERNAL  INHERITANCE  231 

In  cases  in  which  a  foreign  sperm  may  start  develop- 
ment but  take  no  further  part  in  it,  the  resulting  embr>-o 
is  like  the  maternal  race.  Here  we  are  dealing  not  so  much 
with  maternal  inheritance,  but  rather  with  a  special  kind 
of  parthenogenesis.  Such  eggs,  however,  rarely  go  beyond 
the  cleavage  stages. 

The  rate  of  cleavage  of  an  egg  fertilized  by  foreign 
sperm  usually  coincides  with  that  of  the  species  to  which 
the  egg  belongs.  Since  the  cytoplasm  of  the  egg  has,  prior 
to  fertilization,  always  been  under  the  influence  of  its  own 
nucleus,  this  relation  is  what  might  be  expected.  It  is 
necessary  to  study  eggs  from  an  F^  generation  in  such 
cases  in  order  to  judge  how  far  paternal  chromosomes 
may  influence  the  cleavage.  It  is  thinkable,  for  example, 
that  a  spermatozoon  might  bring  in  a  factor  dominant  for 
rate  of  cleavage,  but  because  this  factor  had  not  had  time 
to  influence  the  cytoplasm  its  effect  would  not  show  in  the 
Pi  cross.  In  the  F^,  on  the  other  hand,  the  paternal  char- 
acter might  prove  dominant.  Both  Driesch  and  Boveri 
have  shown  in  the  sea  urchin  that  the  rate  of  cleavage, 
the  pigmentation,  and  the  kind  of  gastrulation  are  entirely 
or  largely  determined  by  the  egg — they  differ  in  opinion 
only  as  to  how  soon  the  influence  of  the  sperm  can  be  seen. 

At  the  time  when  the  larval  skeleton  is  formed  most 
observers  agree  that  the  influence  of  the  foreign  sperm 
makes  itself  felt.  Most  of  the  accounts  of  the  skeleton  of 
hybrid  sea  urchins  describe  it  as  intermediate  in  struc- 
ture, but  one  that  varies  widely  under  different  external 
conditions.  Tennent  has  shown,  in  fact,  that  the  character 
of  the  hybrid  larval  skeleton  is  so  greatly  influenced  by 
the  alkalinity  or  acidity  of  the  sea  water  that  it  can  be 
artificially  thrown  towards  one  or  the  other  side — mater- 
nal or  paternal.  Loeb,  King  and  Moore  have  attempted  to 
determine  whether  the  larval  skeleton  has  dominant  char- 
acters in  certain  parts  and  recessive  ones  in  other  parts. 
They  crossed  the  sea  urchins,  Stronqyloceyitrotus  Fraucis- 
canus  and  S.  purpureus.    Both  the  straight  cross  and  its 


232 


PHYSICAL  BASIS  OF  HEREDITY 


reciprocal  showed  neither  a  great  predominance  of  the 
characters  of  the  paternal  race,  nor  of  the  maternal  race, 
but  rather  certain  characteristic  features  of  purpuratus 
and  others  of  Franciscanns.  The  larval  characters 
appeared  to  be  dominant  or  recessive  taken  singly.  Until 
an  F2  generation  can  be  raised  it  is  obviously  hazardous 
to  speak  here  of  Mendelian  dominance  and  recessiveness 
of  characters  that  are  based  on  F^  observations  alone, 
especially  since  it  is  becoming  more  and  more  apparent 
that  many  F,  characters  are  more  or  less  intermediate, 
and  there  are  no  general  grounds  for  expecting  pure  domi- 
nance or  recessiveness. 

Many  crosses  have  been  made  between  different  species 
of  fish,  and  in  some  of  these  the  young,  at  the  time  of  hatch- 
ing, are  maternal.  It  has  generally  been  supposed  that 
such  cases  are  due  to  the  absorption  of  the  paternal  chro- 
mosomes at  the  first  or  at  later  cleavage  stages.  Loss  of 
chromosomes  has  in  fact  been  recorded  in  several  of  these 
cases  of  maternal  inheritance.  On  the  other  hand,  Miss 
Pinney's  observations,  summarized  in  the  following  table. 

Cross  Development  Results  Chromosomal   Behavior 

Ctenolabrus  9  X  Fundulus  cf"    Development     cases    Early  mitotic  behavior 

during  gastrulation.       is  prevailingly  nor- 
mal. 
Ctenolabrus    9    X    Stenoto- 

mus  o^ Many     hatching     em-    Early  mitoses  are  nor- 

bryos  of  the  mater-        mal. 
nal  type. 
Ctenolabrus   9  X  Menidia  cf"    Advanced  development.  Early  mitoses  are  nor- 
mal. 
Ctenolabrus  cf  X  Fundulus  9    One  hatching   embryo 

reported.    Many  ad- 
vanced     embryos — 
maternal  type. 
Ctenolabrus  cf     X     Stenoto- 

mus  ^      Development       ceases 

during  gastrulation. 
Ctenolabrus  o'   X  Menidia   9    Two  hatching  embryos 

reported.     Maternal 
type. 

show  that  the  maternal  type  may  appear  not  only  when  the 
early  mitoses  are  abnormal,  but  in  one  case  at  least  when 
they  are  normal.    It  is  quite  possible,  therefore,  that  while 


Abnormal  nuclear  be- 
havior occurs. 


Abnormal  mitosis  pre- 
dominant. 

Abnormal  mitosis  is  of 
frequent  occurrence. 


MATERNAL  INHERITANCE  233 

early  loss  of  the  paternal  chromosomes  may  account  for 
some  of  the  cases  of  maternal  embryos,  there  may  also 
be  cases  where  the  chromatin  may  divide  normally  but 
fail  to  produce  any  conspicuous  effects  on  the  cytoy)lasm 
sufficiently  soon  to  become  apparent  in  the  young  fish. 
In  this  connection  the  tobacco  crosses  described  by 
Groodspeed  and  Clausen  may  be  recalled.  In  these  cases 
it  was  a  particular  group  of  chromosomes,  regardless  of 
whether  it  was  of  paternal  or  of  maternal  origin,  whose 
^^ reaction  system'^  dominated  in  the  F,  hybrid. 


CHAPTER  XIX 

THE  PARTICULATE  THEORY  OF  HEREDITY  AND 
THE  NATURE  OF  THE  GENE 

The  attempt  to  explain  biological  phenomena  by  means 
of  representative  particles  has  often  been  made  in  the 
past.  The  superficial  resemblance  of  the  theory  of  the 
gene  to  some  of  the  older  theories,  long  since  abandoned, 
has  furnished  the  opponents  of  the  Mendelian  theory  of 
heredity  an  opportunity  to  injure  the  latter  by  pretending 
that  the  modern  idea  of  the  gene  is  the  same  as  the  older 
ideas  of  Herbert  Spencer  concerning  physiological  units, 
of  Darwin  relating  to  pangenes,  and  especially  of  Weis- 
mann  about  biophors.  There  is  no  need  for  such  con- 
fusion, for  even  a  little  knowledge  of  the  evidence  on  which 
the  old  and  the  new  views  rest  ought  to  have  sufficed  to 
make  evident  some  important  and  essential  differences. 
It  need  not  be  denied,  however,  that  there  is  an  historical 
connection  between  the  mediaeval  theory  of  preformation 
and  the  particulate  theory  of  heredity.  Bonnet,  one  of  the 
best  known  adherents  of  preformation,  believed  at  first 
in  ^' whole '^  germs,  but  later  admitted  that  pieces  of  germs 
might  be  stowed  away  in  regions  of  the  body  likely  to  be 
injured.  Weismann,  also,  the  most  prominent  modem 
adherent  of  preformation,  held  that  whole  germs,  ids,  are 
present  in  the  germ-plasm,  each  standing  for  a  whole 
organism — each  (or  most  or  one  ?)  becoming  unravelled  as 
the  embryonic  development  proceeded.  In  fact,  Weis- 
mann's  entire  theory  was  invented  primarily  to  explain 
embryonic  development  rather  than  genetics.  Its  connec- 
tion with  the  modern  idea  of  the  germ-plasm  is  little  more 
than  an  analogy — for  reduction  in  Weismann 's  original 

234 


PARTICULATE  THEORY  OF  HEREDITY    235 

sense  meant  the  sorting  out  of  the  wholes  of  ancestral 
germ-plasms  with  which  he  peopled  the  chromosomes.^ 

The  danger  of  any  appeal  to  a  theory  of  representative 
particles  obviously  lies  in  the  ease  with  which  by  its  means 
any  phenomenon  might  be  accounted  for,  if  the  theorizer 
is  allowed  to  endow  the  particles  with  any  and  all  tlie 
attributes  that  he  wishes  to  use  in  his  explanation.  It 
was  because  Bonnet,  Spencer,  and  AYeismann  assigned 
arbitrarily  attributes  to  the  ultimate  particles  of  living 
matter,  that  these  views  appear  to-day  highly  speculative. 
The  different  kind  of  evidence  to  which  the  modern  theory 
of  the  gene  appeals  is  what  I  wish  to  emphasize  here. 

The  Evidence  for  the  Gene 

The  evidence  that  Mendelian  inheritance  rests  on  the 
distribution  of  separate  elements  has  already  been  given. 
The  numerical  results  obtained  in  the  second  generation 
from  any  Mendelian  cross  involving  a  pair  of  contrasted 
characters,  find  their  explanation  on  the  assumption  that 
the  two  original  germ-plasms  (or  some  element  in  them) 
separate  cleanly  in  the  germ-cells  of  the  F^  hybrid.  Tested 
by  back-crossing  the  assumption  is  verified.  Recombining 
the  Pj,  F^,  F2  individuals  in  all  possible  ways  also  gives 
results  consistent  with  the  very  simple  assumption  that 
whatever  it  is  that  causes  one  race  to  produce  one  charac- 
ter, and  another  race  another  character,  the  two  separate 
in  the  hybrid  in  such  a  way  that  equal  numbers  of  germ- 
cells  of  each  kind  are  produced.  Up  to  this  point  the 
results  do  not  tell  us  whether  the  two  germ-plasms  separ- 
ate as  wholes — one  from  the  other — or  whether  only  some 
part  or  parts  behave  in  this  way.    But  when  two  or  more 

^The  nominal  adoption  (1904)  toward  the  end  of  his  career  of  heredi- 
tary units  in  the  Mendelian  sense  did  not  go  deep.  Weismann  still  adhered 
to  his  view  of  dissociation  of  the  ids  as  their  most  characteristic  feature— 
the  only  one  in  fact  for  which  they  were  originally  invented.  The  e>'idence 
on  which  Mendelian  units  rest  has  nothing  whatever  to  do  with  this 
cardinal  doctrine  of  Weisraann's  teaching. 


236  PHYSICAL  BASIS  OF  HEREDITY 

pairs  of  contrasted  characters  are  involved  in  the  same 
cross,  we  get  further  information  as  to  the  situation. 

For  example,  Mendel  showed  that  when  peas  that  are 
both  yellow  and  round  are  crossed  to  peas  that  are  both 
green  and  wrinkled,  there  appear  in  the  F^  generation  not 
only  the  original  combinations,  but  also  recombinations  of 
these,  viz.,  yellow  and  wrinkled;  and  green  and  round 
(Fig.  10()).  Here  also  the  numerical  results  9:3:3:1 
can  be  explained  on  the  theory  that  the  representatives 
of  each  pair  of  characters  separate  in  the  germ-plasm, 
and  that  the  separation  of  each  pair  is  independent  of 
what  takes  place  in  the  other  pair.  Obviously  it  can  no 
longer  be  whole  germ-plasms  that  separate,  but  there 
must  be  different  pairs  of  elements  in  the  germ-plasm  that 
assort  independently  of  each  other.  It  has  been  found  that 
this  principle  of  independent  assortment  may  apply  to  a 
considerable  number  of  pairs  of  characters  segregating  at 
the  same  time.  The  only  restriction  that  is  found  is  in 
the  case  of  linked  pairs  of  characters.  This  relation  will 
be  considered  later. 

The  independent  assortment  of  the  pairs  of  characters 
proves  that  the  elements  that  stand  for  the  characters  in 
the  two  original  germ-plasms  may  separate  from  each 
other.  If  each  such  pair  of  characters  represented  one 
of  the  pairs  of  homologous  chromosomes,  the  evidence,  so 
far  considered,  would  be  in  accord  with  the  view  that  the 
chromosomes  were  the  ultimate  units  involved  in  the  proc- 
esses of  segregation  and  assortment.  The  chromosomes 
are,  as  has  been  shown,  independent  units  in  the  germ- 
plasm.  But  as  Drosophila  shows,  there  are  many  more 
pairs  of  characters  than  there  are  pairs  of  chromosomes. 

It  is  obvious  that  if  the  chromosomes  are  the  ulti- 
mate units  involved,  and  remain  intact,  there  could  be  no 
more  independent  pairs  of  characters  than  there  are  pairs 
of  chromosomes.  In  animals  and  in  plants  there  are 
no  cases  known  where  there  are  more  independent  pairs 
than  there  are  chromosomes,  so  that,  as  has  been  pointed 


PAETICULATE  THEORY  OF  HEREDITY    237 

out  in  another  connection,  this  evidence  may  also  be 
appealed  to  as  favorable. 

The  behavior  of  linked  pairs  shows,  however,  that  the 
analysis  must  be  carried  further,  because,  despite  linkas^e, 
the  elements  that  went  in  together  may  be  separated.  The 
evidence  shows  that  while  some  linked  genes  separate 
almost  as  freely  as  do  independent  genes,  so  that  their 
linkage  to  each  other  can  only  be  safely  determined  by 
their  relation  to  certain  other  genes,  other  linked  genes 
may  separate  not  once  in  a  hundred  times,  or  even  less 
often.  Between  these  extremes  all  intermediate  linkage 
values  are  found.  These  results  indicate  that  the  chromo- 
somes do  not  represent  the  ultimate  elements  that  may  be 
separated  out  of  the  original  complex  (germ-plasm). 

We  are  led,  then,  to  the  conclusion  that  there  are  ele- 
ments in  the  germ-plasm  that  are  sorted  out  independently 
of  one  another.  The  Brosophila  evidence  shows  at  least 
several  hundred  independent  elements,  and  as  new  ones 
still  appear  as  frequently  as  at  first,  the  indications 
are  that  there  are  many  more  such  elements  than  those 
as  yet  identified. 

These  elements  we  call  genes,  and  what  I  wish  to  insist 
on  is  that  their  presence  is  directly  deducible  from  the 
genetic  results,  quite  independently  of  any  further 
attributes  or  localizations  that  we  may  assign  to  them. 
It  is  this  evidence  that  justifies  the  theory  of  partic- 
ulate inheritance. 

So  far  as  representative  elements  in  the  germ-plasm 
are  concerned,  we  might  be  content  to  rest  the  case  on  the 
preceding  analysis  of  the  results ;  but  recent  work  has  now 
advanced  far  enough  to  tempt  us  to  assign  further  attri- 
butes to  the  genes  than  those  deducible  from  the  preceding 
analysis  alone.  Some  of  these  attributes  may  appear 
better  established  than  others,  but,  all  together,  they  give 
a  consistent  body  of  data,  and  have  therefore  a  certain 
value  and  use. 


238  PHYSICAL  BASIS  OF  HEEEDITY 

It  has  been  pointed  out  that  the  evidence  shows  not 
only  that  the  genes  are  carried  by  the  chromosomes,  but 
that  there  may  be  interchanges  between  paternally- 
derived  and  maternally-derived  chromosome  pairs.  The 
evidence  shows  that  this  interchange  is  a  normal  feature 
of  the  germ-cell,  and  not  peculiar  to  hybrids,  or  to  a 
heterozygous  condition  of  the  pairs. 

This  analysis  leads  then  to  the  view  that  the  gene  is 
a  certain  amount  of  material  in  the  chromosome  that  may 
separate  from  the  chromosome  in  which  it  lies,  and  be 
replaced  by  a  corresponding  part  (and  by  none  other) 
of  the  homologous  chromosome.  It  is  of  fundamental  sig- 
nificance in  this  connection  to  recognize  that  the  genes 
of  the  pair  that  interchange  do  not  jump  out  of  one  chro- 
mosome into  the  other,  so  to  speak,  but  are  changed 
by  the  thread  breaking  as  a  piece  in  front  of  or  else 
behind  them,  but  not  in  both  places  at  once,  as  would 
be  the  case  if  only  a  single  pair  of  allelomorphs  were 
involved  each  time. 

That  the  gene  does  not  stand  for  the  whole  length  of 
the  chromosome  between  two  other  known  genes  is  shown 
by  the  fact  that  new  genes  arising  by  mutation  in  the  inter- 
mediate region  do  not  affect  the  character  of  the  gene 
already  known.  This  fact  recurring  continually  in  Droso- 
phila,  where  new  mutations  frequently  appear,  reassures 
us  that  the  idea  of  the  gene  as  a  very  small  part  of  the 
thread  is  a  legitimate  conclusion,  even  if  we  can  not  tell 
how  large  or  how  small  that  region  is. 

1.  The  Manifold  Effects  of  Each  Gene 
If  we  examine  almost  any  mutant  race,  such  as  the 
race  of  white -eyed  Drosophila,  we  find  that  the  white  eye 
is  only  one  of  the  characteristics  that  such  a  mutant  race 
shows.  The  productivity  of  the  individual  is  also  much 
affected,  and  the  viability  is  lower  than  in  the  wild  fly.  AU 
of  these  peculiarities  are  found  whenever  the  white  eye 
emerges  from  a  cross,  and  are  not  separable  from  the 


PARENTS 


A 

^ 


Fig.   106. — Diagram  to  show  the  inheritance  of  two  pairs  of  Mendelian  characters,  viz. 
yellow  versus  green  peas,  and  round  versus  wrinkled  skin  in  garden  peas. 


PAETICULATE  THEOEY  OF  HEREDITY    239 

white-eyed  condition.  It  follows  that  whatever  it  is  in 
the  germ-plasm  that  produces  white  eyes,  also  produces 
other  modifications  as  well,  and  modifies  not  only  such 
"superficial"  things  as  color,  but  also  such  '* fundamen- 
tal ^ '  things  as  productivity  and  viability.  Many  examples 
of  this  manifold  effect  are  known  to  students  of  heredity. 
It  is  perhaps  not  going  too  far  to  say  that  any  change 
in  the  germ-plasm  may  produce  many  kinds  of  effects  on 
the  body.  Clearly  then  the  character  that  we  choose  to 
follow  in  any  case  is  only  the  most  conspicuous  or  (for 
purposes  of  identification)  the  most  striking  or  convenient 
modification  that  is  produced.  Since,  however,  these 
effects  always  go  together,  and  can  be  explained  by  the 
assumption  of  a  single  unit  difference  in  the  germ-plasm, 
the  particular  difference  in  the  germ-plasm  is  more  sig- 
nificant than  the  character  chosen  as  its  index. 

2.  The  Variability  of  the  Character  is  Not  Due  to  the 

Corresponding  Variability  of  the  Gene 

All  characters  are  variable,  but  there  is  at  present 
abundant  evidence  to  show  that  much  of  this  variability 
is  due  to  external  conditions  that  the  embryo  encounters 
during  its  development.  Such  differences  as  these  are  not 
transmitted  in  kind — they  remain  only  so  long  as  the 
environment  that  produces  them  remains.  By  inference 
the  gene  itself  is  stable,  although  the  character  varies ;  yet 
this  point  is  very  difficult  to  establish.  The  evidence  is 
becoming  stronger  nevertheless  that  the  germ-plasm  is 
relatively  constant,  while  the  character  is  variable. 

3.  Characters  That  are  Indistinguishable  May  be  the 

Product  of  Different  Genes 

We  find,  in  experience,  that  we  cannot  safely  infer 
from  the  appearance  of  the  character  what  gene  is  pro- 
ducing it.  There  are  at  least  three  white  races  of  fowls, 
produced  by  different  genes.    We  can  synthesize  white- 


240  PHYSICAL  BASIS  OF  HEREDITY 

eyed  flies  that  are  somatically  indisting-uisiiable  from  the 
ordinary  white-eyed  race,  yet  they  are  the  combined  prod- 
uct of  several  known  color-producing  genes.  The  purple 
eye  color  of  Drosophila  is  practically  indistinguishable 
from  the  eye  colors  maroon  and  garnet.  In  a  word,  we  are 
led  again  to  units  in  the  germ-plasm  in  our  final  analysis 
rather  than  to  the  appearance  of  a  character. 

4.  Inference  That  Each  Character  is  the  Product  of 

Many  Genes 

We  find  that  any  one  organ  of  the  body  (such  as  an 
eye,  leg,  wing)  may  appear  under  many  forms  in  different 
mutant  races  as  a  result  of  changes  of  genes  in  the  germ- 
plasm.  It  is  a  fair  inference,  I  think,  that  the  normal 
units — ^the  allelomorphs  of  the  mutant  genes — also  often 
affect  the  same  part.  We  have  found  about  50  different 
factors  that  affect  eye-color,  15  that  affect  body-color,  and 
at  least  10  factors  for  length  of  wing  in  Drosophila. 

If,  then,  it  is  a  fair  inference  that  the  units  in  the  wild 
fly,  that  behave  as  Mendelian  mates  to  the  mutant  genes, 
also  affect  the  same  organ  that  the  mutant  gene  affects,  it 
follows  that  many  genes,  and  perhaps  a  very  large  num- 
ber, are  involved  in  the  production  of  each  organ  of  the 
body.  It  might  perhaps  not  be  a  very  great  exaggeration 
to  say  that  every  gene  in  the  germ-plasm  affects  several 
or  many  parts  of  the  body ;  in  other  words,  that  the  whole 
germ-plasm  is  instrumental  in  producing  each  and  every 
part  of  the  body. 

Such  a  statement  may  seem  at  first  hearing  to  amount 
almost  to  an  abandonment  of  the  particulate  conception 
of  heredity,  but  on  the  contrary,  the  statement  conveys  a 
very  important  idea  in  the  modern  conception  of  the 
nature  of  the  genes  and  the  way  they  act. 

The  essential  point  here  is  that  even  although  each  of 
the  organs  of  the  body  may  he  largely  a  product  of  the 
entire  germ-plasm,  yet  this  germ-plasm  is  made  up  of 
units  that  are  independent  of  each  other  in  at  least  two 


PAKTICULATE  THEORY  OF  HEREDITY    241 

respects,  viz.,  in  that  each  one  may  change  {mutate)  with- 
out the  others  changing,  and  in  segregation  and  in  crossing 
over  each  pcdr  is  separable  from  the  others. 

5.  "The  Organism  as  a  Whole,"  or  The  Collective 

Action  of  the  Gtenes 

Several  writers  have  stated  their  objections  to  the 
particulate  theory  of  heredity  on  the  grounds  of  their 
belief  that  the  organism  is  a  "whole."  If  this  phrase  is 
intended  to  mean  that  there  is  some  sort  of  an  entity  or 
entelechy  that  directs  all  processes  that  go  on  in  each 
living  thing,  there  is  little  to  be  said  here,  except  that 
this  very  old  idea  has  not  been  found  profitable  as  a 
working  hypothesis.  It  is  improbable,  however,  that 
many  biologists  mean  to  appeal  to  any  such  vitalistic 
agency  when  they  speak  of  the  "organism  as  a  whole," 
but  have  rather  some  other  idea  in  mind.  I  am  inclined  to 
think  that  certain  phenomena  of  embryonic  development 
are  responsible  for  the  slogan  of  the  "organism  as  a 
whole. ' '  In  the  segmentation  of  the  egg  the  entire  chromo- 
somal complex  is  distributed  to  every  cell  in  the  body. 
Each  cell  inherits  the  whole  germ-plasm.  How  then  it 
may  be  asked  can  the  result  depend  on  the  particular 
make-up  of  its  chromosomes  rather  than  on  the  action  of 
the  whole  material! 

G-ranted  that  we  know  very  little  about  the  interactions 
between  the  cells  that  cause  some  of  them  to  differentiate 
in  one  direction,  others  in  other  directions,  yet  if  one  fer- 
tilized egg  should  begin  its  development  with  one  kind  of 
material,  and  another  egg  with  a  different  material,  should 
we  not  expect  the  end  products  to  be  different,  irrespective 
of  the  way  in  which  the  materials  were  present  in  the 
original  egg^.  No  matter  where  the  differences  may  lie, 
i.e.,  whether  in  the  nucleus  or  in  the  cytoplasm,  there  is 
nothing  here  in  any  way  inconsistent  with  this  particulate 
theory  of  the  composition  of  the  germ-plasm.  On  the 
contrary,  the  only  conclusion  that  seems  at  all  reasonable 

16 


242  PHYSICAL  BASIS  OF  HEEEDITY 

is  that  if  differences  are  present  at  the  beginning,  the  end 
product  is  expected  to  be  correspondingly  different.  So 
much  is  clear.  But  why,  it  may  still  be  asked,  are  not  two 
organisms  that  are  diff'erent  at  the  start,  if  only  in  some 
one  difference,  different  later  in  every  part,  rather  than 
in  only  some  one  small  part  such  as  in  a  red  or  in  a  white 
eye.  The  answer  is,  of  course,  that  the  first  difference 
was  such  that  it  affected  principally  a  particular  process, 
viz.,  the  formation  of  the  red  pigment  of  the  eye,  and  to 
a  less  degree,  or  not  at  all,  other  chemical  processes.  This 
seems  to  me  an  entirely  consistent  view. 

Perhaps  the  difficulty  in  accepting  the  particulate 
theory  lies  in  the  erroneous  idea  that  the  specific  effect 
comes  into  action  only  at  the  moment  when  the  red  pig- 
ment is  about  to  form.  But  no  one  has,  so^  far  as  I  know, 
made  such  a  claim.  It  may  be  true,  but  it  has  not  been 
proven,  and  is  moreover  not  in  any  way  essential  to  the 
assumption  of  the  particulate  theory.  On  the  contrary, 
as  our  knowledge  of  Mendelian  heredity  has  increased 
many  cases  have  been  found  where  a  special  factor-differ- 
ence affects  not  only  one  part  of  the  body  but  many  parts. 
It  is  true  that  the  particulate  theory  as  held  at  one  time 
by  Eoux  and  for  a  long  time  by  Weismann  was  used  to 
explain  the  differentiating  changes  in  the  segmenting 
egg  and  embryo  in  the  sense  that  development  was  looked 
upon  as  a  process  that  resulted  immediately  in  the  sorting 
out  of  the  inherited  chromosomal  particles  to  the  differ- 
ent parts  of  the  organism.  Differentiation  resulted  in  the 
sorting  out  of  particular  genes  to  particular  groups  of 
cells  whose  development  they  controlled.  But  the  cyto- 
logical  evidence  in  regard  to  the  chromosomes  gave  no 
evidence  in  support  of  the  view,  and  the  evidence  from 
the  experimental  study  of  embryology  seemed  to  entirely 
disprove  any  such  basis  for  the  developmental  phe- 
nomena. In  fact,  Eoux  himself  abandoned  this  view 
in  the  light  of  the  brilliant  experiments  of  Driesch  and 
of  other  embryologists. 


PAETICULATE  THEORY  OF  HEREDITY    243 

Our  present  conception  of  the  relation  of  the  germ- 
plasm  to  developmental  phenomena  has  then  only  a  most 
superficial  resemblance  to  the  older  theories.  The  newer 
point  of  view  may  be  summed  up  in  a  few  words,  and  has 
in  fact  been  stated  already.  First,  that  each  gene  may 
have  manifold  effects  on  the  organism,  and  second,  that 
every  part  of  the  body,  and  even  each  particular  character, 
is  the  product  of  many  genes.  The  evidence  for  these  two 
conclusions  has  been  so  repeatedly  referred  to  in  the  pre- 
ceding pages  that  it  is  not  necessary  to  go  over  it  again, 
but  it  may  be  worth  while  to  emphasize  that  these  two 
conclusions  are  not  pure  speculations,  but  derived  from 
the  evidence  itself.  It  may  also  be  well  to  point  out  that 
even  if  the  whole  germ-plasm — the  sum  of  all  the  genes — 
acts  in  the  formation  of  every  detail  of  the  body,  still 
the  evidence  from  heredity  shows  that  this  same  material 
becomes  segregated  into  tw^o  parts  during  the  maturation 
of  the  egg  and  sperm,  and  that  at  this  time  individual 
elements  separate  from  each  other  largely  independently 
of  the  separation  of  other  pairs  of  elements.  It  is  in  this 
sense,  and  in  this  sense  only,  that  we  are  justified  in  speak- 
ing of  the  particulate  composition  of  the  germ-plasm  and 
of  particulate  inheritance. 

There  is  a  further  idea  deducible  from  well-kno\\Ti 
facts  of  physiology  that  may  at  first  sight  seem  to  give 
an  impression  that  the  organism  is  a  *' whole.''  This 
is  the  action  of  one  part  of  the  body  on  other  parts  by 
means  of  substances  set  free  in  the  blood,  called  hor- 
mones. Many  of  them  arise  through  the  action  of  certain 
so-called  endocrine  glands.  But  the  relation  here  is  so 
obviously  different  from  the  problem  dealt  with  as  par- 
ticulate inheritance  that  it  calls  for  little  more  than 
passing  notice.  It  may,  however,  not  be  without  interest 
to  refer  to  one  case  of  the  kind  in  which  an  endocrine 
secretion  depends  on  a  genetic  factor  inherited  in  the 
same  way  as  are  other  genetic  factors.  There  is  a  race 
of  poultry  known  as  Sebrights  (Fig.  107,  a)  in  which  the 


244  PHYSICAL  BASIS  OF  HEREDITY 

males  are  always  hen-feathered.  This  means  that  the 
feathers  of  the  neck  and  back  and  the  tail  coverts  of  the 
Sebright  cock  are  nearly  like  those  of  the  hen  of  this 
breed,  and  not  long  and  pointed  as  in  the  ordinary  cock. 
AYhen  Sebrights  are  crossed  to  game  bantams  (which 
have  ordinary  males),  the  F^  males  are  hen-feathered. 
When  these  are  inbred  the  two  types  reappear  in  the  Fo 
males.  One,  or  probably  two,  Mendelian  factor  differ- 
ences account  for  the  results. 

It  has  been  sho^^m  that  when  the  testes  are  removed 
from  the  Sebright  male,  he  then  develops  at  the  next  moult 
(or  at  once  if  some  feathers  are  plucked  out)  the  long 
and  highly  colored  feathers  of  the  ordinary  male  (Fig. 
107,  h).  It  is  probable,  therefore,  that  the  testes  of  the 
Sebright  produce  an  internal  secretion  that  inhibits  in  the 
male  the  full  development  of  certain  feathers.  This  makes 
him  like  the  hen,  and  in  this  connection  it  is  interesting  to 
note  that  when  the  ovary  of  a  hen  of  an  ordinary  breed  is 
removed  she  also  develops  the  full  plumage  of  the  cock,  as 
Goodale  has  clearly  demonstrated.  Whether  the  testes  of 
a  male  are  of  the  sort  to  develop  this  inhibiting  substance, 
depends  on  the  presence  in  the  cells  of  the  testes  of  certain 
genetic  factors.  These  factors  are  present,  presumably, 
in  all  the  cells  of  the  body,  but  if  they  are,  their  activity 
is  ineffective  in  the  absence  of  secretions  produced  by  the 
testes,  as  is  shown  by  the  castrated  Sebright  becoming 
cock-feathered.  Whether  this  substance  belongs  in  the 
heterogeneous  group  of  substances  called  hormones — 
defined  by  the  kind  of  action  they  produce  rather  than  by 
any  chemical  peculiarity — or  to  the  groups  of  enzymes 
that  have  a  more  or  less  specific  action,  caimot  be  stated. 

The  fore^^oing  discussion  touches  upon  the  question  as 
to  whether  there  is  any  evidence  that  the  genes  themselves 
are  to  be  regarded  as  enzymes.*     In  almost  all  of  the 

*  Inadequate  as  is  our  knowledge  of  the  physico-chemical  processes  that 
go  on  in  development,  it  is  enough  to  indicate  that  many  processes  are 
at  work. 


PARTICULATE  THEORY  OF  HEREDITY    245 

recent  papers  (Beijerinck,  Riddle,  Goldschmldt)  that 
touch  on  this  question  it  is  argued,  from  the  evidence  of  the 
specific  enzymes  supposed  or  demonstrably  involved  in 
the  production  of  some  final  stage  in  the  chemical  reaction 
that  leads  to  the  character  in  question,  that  the  gene  itself 
is  the  same  specific  enzyme.  The  argument  shifts  back 
and  forth  from  unit-character  to  unit-factor.  The  reason- 
able position  to  take  in  this  matter  is,  in  my  opinion,  that 
stated  by  Loeb  and  Chamberlain  (1915),  ''The  hereditary 
factor  in  this  case  must  consist  of  material  which  deter- 
mines the  formation  of  a  given  mass  of  these  enzymes, 
since  the  factors  in  the  chromosomes  are  too  small  to  carry 
the  whole  mass  of  the  enzymes  existing  in  the  embryo 
or  adulf  It  should  not  be  forgotten,  however,  that  the 
evidence  in  favor  of  enzyme  action  as  the  most  important 
developmental  process  is  by  no  means  established,  and 
even  were  the  evidence  for  this  view  adequate,  the  stages 
between  such  action  and  the  ultimate  chemical  nature  of 
the  gene  may  be  too  great  to  be  cleared  at  a  single  bound. 
Some  of  the  modern  work  on  the  chemical  composition  of 
the  nucleus  indicates  that  extremely  complex  protein  com- 
pounds may  be  present  in  it^ — even  though  some  of  the 
split  products  obtainable  from  it  may  be  relatively  simple. 
It  seems  to  me  therefore  that  it  is  both  premature  and 
highly  speculative  at  present  to  tie  up  the  genetic  evi- 
dence concerning  the  genes  with  hypotheses  concerning 
their  chemical  composition.  I  urge  this,  but  at  the  same 
time  I  realize  of  course  that  we  should  endeavor  to  obtain 
as  soon  as  possible  better  knowledge  as  to  the  chemical 
nature  of  the  .chromatin. 

Another  question  concerning  the  gene,  that  has  been 
raised,  is  whether  it  is  to  be  regarded  as  something  having 
a  definite  molecular  constitution,  or  whether  the  gene  is  to 
be  regarded  as  a  quantity  of  material  fluctuating  about  a 
mode— its  definiteness  representing  only  a  general  ten- 
dency for  the  same  frequency  distribution  to  recur  in 
each  species.    From  the  nature  of  the  case  such  a  question 


246  PHYSICAL  BASIS  OF  HEREDITY 

is  speculative,  and  would  have  little  importance  were  it  not 
that,  by  imputing  to  the  advocates  of  Mendelian  heredity 
the  assumption  of  absolute  fixity  to  the  gene,  attempts 
have  been  made  to  throw  the  burden  of  proof  that  the 
genes  are  ^'constant''  on  the  advocates  of  Mendelism. 

So  far  as  the  genetic  evidence  is  involved,  I  see  at  pres- 
ent no  way  of  deciding  whether  the  gene  has  a  definite 
molecular  constitution,  or  is  only  something  that  fluctuates 
under  the  condition  of  its  occurrence  about  a  mode.  Inter- 
esting as  it  might  be  to  speculate  about  these  alternatives, 
it  seems  futile  to  do  so  at  present,  but  there  is  one  impli- 
cation that  I  should  like  to  examine.  If  the  gene  is  a 
chemical  molecule  it  is  not  evident  how  it  could  change 
except  by  altering  its  chemical  constitution.  Its  influence, 
i,e.,  the  chemical  effects  it  produces,  might,  however,  be 
altered  by  changing  other  substances  with  which  the  mate- 
rial it  produces  reacts.  This  is  the  idea  involved  in  the 
theory  of  ** modifying  genes.'' 

But  if  the  gene  is  a  fluctuating  amount  of  something  it 
might  seem  that  any  *' fluctuation''  that  is  present  at  one 
time  might  be  perpetuated  by  selection,  and  that  a  further 
fluctuation  in  the  same  direction  might  be  utilized  for  a 
further  advance,  etc.  It  may  be  pointed  out  that  this 
picture  of  the  process  is  quite  fanciful,  and  its  success 
would  depend  largely  on  a  denial  of  the  premise  as  to  the 
nature  of  the  gene,  viz.,  that  it  is  of  a  fluctuating  amount. 
Johannsen's  facts  contradict  an  interpretation  of  the 
fluctuations  of  the  character  being  due  to  a  new  modal 
position  of  the  gene  standing  for  that  character.  And  his 
facts  furnish  the  only  crucial  evidence  we  have  at  present. 


B 


D 


Fig.  107. — A.  Adult  hen-feathered  Campine  male.  B.  Adult  male  of  same  race  that  had  been 
castrated  while  still  a  young  bird.  When  it  became  older  it  developed  cock-feathering.  It  resembles 
the  male  of  another  race  of  Campines  in  which  the  male  is  normally  cock-feathered.  C.  Adult  hen- 
feathered  Sebright  male.  D.  Adult  male  Sebright,  that  had  been  castrated  while  still  a  younj;  bird.  It 
developed  cock-feathering  when  it  became  older. 


CHAPTER  XX 
MUTATION 

Concerning  the  origin  of  the  germinal  differences  that 
give  rise  to  mutant  characters  very  little  is  known  at  pres- 
ent except,  (1)  that  they  appear  infrequently,  (2)  that  the 
change  is  definite  from  the  beginning,  (3)  that  some  of 
the  changes  at  least  are  recurrent,  and  (4)  that  the  differ- 
ence between  the  old  character  and  the  new  one  is  small 
in  some  cases  and  greater  in  others.  I  do  not  think  that 
any  of  the  work  purporting  to  produce  specific  mutational 
changes  has  succeeded  in  establishing  its  claims,  at  least 
in  the  sense  that  we  can  pretend  at  present  to  control  the 
appearance  of  specific  mutant  changes,  and  until  this  is 
done  we  can  not  hope  to  find  out  very  much  as  to  the 
nature  of  these  changes.  Our  study  of  the  germ-plasm 
is  largely  confined,  therefore,  for  the  present,  to  a  study 
of  transmission  of  the  genes,  to  the  kinds  of  effects  they 
produce  on  the  organism,  and  to  the  special  relations  of 
the  genes  in  the  chromosomes  where  they  are  located. 

Concerning  the  frequency  of  mutation  there  is  a  slowly 
increasing  body  of  evidence  showing  in  some  animals 
and  plants  how  often  or  how  rarely  changes  of  this  kind 
take  place.  The  impression  prevails  that  mutation  is  less 
rare  in  some  species  than  in  others,  and  while  I  am  inclined 
to  think  that  this  may  be  true,  not  much  value  can  be 
ascribed  to  such  impressions ;  for  it  is  not  improbable  that 
the  frequency  with  which  mutations  are  found  is  often 
directly  in  proportion  to  the  number  of  individuals  exam- 
ined and  to  familiarity  with  the  type  in  question,  so 
that  the  smaller  changes  are  not  overlooked.  The  dis- 
covery of  new  mutant  types  in  almost  every  plant  and 
animal  that  has  been  carefully  examined  indicates  at  least 
the  very  general  occurrence  of  definite  mutations,  and  the 

247 


248  PHYSICAL  BASIS  OF  HEREDITY 

great  variety  of  types  shown  by  nearly  all  of  our  domesti- 
cated animals  and  plants — varieties  that  follow  MendePs 
law — appears  to  give  further  support  to  the  view  that  the 
process  of  mutation  is  widespread. 

One  of  the  most  interesting  phenomena  connected  with 
mutation  is  the  recurrence  of  the  same  change.  It  has 
long  been  recognized  that  certain  *^ sports''  such  as  albi- 
nos and  melanic  forms  are  found  again  and  again  in 
nature.  In  insects  there  are  many  records  of  the  sporadic 
appearance  of  the  same  type,  such  as  the  light  form  (lacti- 
color)  of  the  moth  Abraxas.  It  is  true  that  not  all  such 
appearances  are  to  be  accepted  offhand  as  the  first  appear- 
ance of  the  mutative  change,  since  when  these  are  reces- 
sive it  is  probable  in  most  cases  *  that  the  actual  mutation 
occurred  several  generations  before  the  mutated  genes 
came  together  to  produce  the  mutant  character.  But 
granting  this,  it  is  at  least  probable  that  the  same  type 
has  appeared  in  many  cases  independently.  The  only 
evidence  that  can  be  relied  upon  in  such  cases  is  from 
pedigreed  cultures,  followed  up  by  evidence  that  the 
mutants  that  look  alike  are  really  due  to  mutations  in  the 
same  locus.  Fortunately  there  is  actual  evidence,  both 
for  plants  and  for  animals,  that  can  be  appealed  to  to  show 
that  the  same  mutations  recur. 

The  most  extensive  evidence  is  from  Drosophila 
melanogaster.  One  of  the  first  mutants  that  appeared, 
viz.,  white  eyes,  has  appeared  anew  in  our  cultures  about 
three  times,  in  cultures  known  to  be  free  from  it  before  and 
not  contaminated.  The  same  mutant  has  been  found  by 
several  other  observers.  The  eye-color  vermilion  has 
appeared  at  least  six  times;  the  wing  character  called 
rudimentary,  five  times;  cut  wing  has  been  found  four 
times;  truncate  wing  has  frequently  appeared,  but  has 
not  necessarily  been  always  produced  by  the  same  change. 
Certain    characters    such    as    notch    wings,    that    have 


*  Recessive  mutations  in  the  X-chromosomes  of  the  XX-XY  type  may 
appear  in  the  male  in  the  next  generation. 


MUTATION  249 

appeared  quite  often,  represent,  it  seems,  a  pecuHar 
change  whose  relation  to  the  changes  that  stand  behind 
other  mutant  characters  is  not  yet  worked  out. 

In  plants  the  best  evidence  is  that  reported  by  Emerson 
for  Indian  corn.  Emerson  has  shown  that  when  a  race 
of  corn  {Zea  mais)  having  red  cobs  and  red  seeds  is 
crossed  to  a  race  having  white  cobs  and  white  seeds  only, 
the  two  original  combinations  appear  in  the  second  (Fo) 
generation  giving  plants  ^vith  red  cobs  and  red  seeds  and 
plants  with  white  cobs  and  white  seeds.  Either  a  single 
factor  determines  that  both  cob  and  seed  are  red  in  one 
case  and  white  in  the  other,  or  if  the  color  of  each  part 
is  due  to  a  separate  factor  these  factors  are  completely 
linked.  Now  striped  seeds  with  white  cobs  sometimes 
mutate  to  red  seeds  and  red  cobs.  The  new  combination 
(red  and  red)  acts  as  a  unit  toward  the  other  kno^vn  com- 
binations. Therefore  a  single  factor  must  have  changed, 
for,  if  not,  mutation  must  occur  in  two  (or  more)  closely 
linked  factors,  i.e.,  for  seed  and  cob  color  at  the  same  time, 
which  is  highly  improbable. 

In  forms  propagating  by  sexual  methods  it  cannot 
be  told  whether  mutation  has  occurred  in  one  locus  or  in 
both  homologous  loci  at  the  same  time,  because  in  the  Qgg 
one  of  each  pair  of  genes  is  lost  in  the  polar  body,  and 
irrespective  of  whether  one  or' two  mutated  genes  were 
present  only  one  member  of  the  pair  is  left  in  the  ripe  Qgg ; 
and  in  the  sperm  the  chance  of  any  one  sperm  reaching 
the  egg  is  so  small  that  it  is  unlikely  that  the  difference 
between  one  sperm  or  two  sperms  having  the  mutated 
locus  could  be  detected.  It  is  true  that  of  the  twelve  domi- 
nant mutants  that  have  appeared  in  Drosophila  each 
appeared  at  first  in  a  single  individual — never  two — which 
might  appear  to  favor  the  single  locus  view,  but  this  evi- 
dence is  too  meagre  to  be  significant.  Mutants  from  reces- 
sive genes  usually  come  to  light  in  about  a  quarter  of  the 
offspring  of  a  given  pair.  This  means  that  both  parents 
were  heterozygous  for  the  mutant  gene,  but  this  gene 


250  PHYSICAL  BASIS  OF  HEREDITY 

must  have  arisen  at  least  one  generation  earlier,  and 
have  been  carried  over  into  the  two  heterozygous  indi- 
\dduals  in  question. 

It  would  be  a  point  of  capital  importance  if  it  could  be 
determined  beyond  doubt  that  at  times  recessive  mutant 
genes  change  back  to  the  original  (wild  type)  gene,  or 
even  if  a  recessive  gene  could  mutate  to  a  dominant  one. 
The  appearance  of  the  wild  type  in  a  pure  culture  of  a 
mutant  race  can  be  accepted  as  good  evidence  of  such  a 
change  only  when  every  possibility  of  contamination  by 
the  wild  type  is  excluded,  and  this  is  difficult  to  regulate. 
In  our  cultures  we  have  come  across  such  cases,  but  have 
not  ventured  to  exploit  them,  since  wild-type  flies  are 
always  present  in  the  laboratory,  and  hence  the  discovered 
form  may  have  arisen  through  accidental  contamination. 
Thus  even  when  a  red-eyed  yellow  fly  appeared  in  the 
white-eyed  yellow  stock  there  is  the  barest  chance  that  a 
yellow  red-eyed  fly,  or  an  egg  of  such  a  fly,  had  somehow 
gotten  into  the  stock.  Certainty  can  be  attained  only  when 
a  stock,  pure  for  several  mutant  characters,  reverts  to  the 
normal  in  one  of  these  characters,  and  not  in  the  others. 
Only  one  case  of  this  kind  that  is  above  suspicion  has  been 
as  yet  recorded.  This  is  a  mutant  stock  in  which,  as  May 
has  recorded,  reversion  to  the  wild  type  occurs  with  such 
frequency  that  there  can  be  no  chance  of  error.  The  stock 
in  question,  bar  eye,  is  a  dominant  mutant  and  the  rever- 
sion therefore  is  to  the  recessive  wild  type  of  eye  (round 
eye).  The  change  back  to  normal  is  complete,  since  such 
individuals  give  only  normal  offspring.  When  such  a 
mutant  chromosome  comes  from  the  mother  and  goes  into 
a  son  he  has  normal  (wild  type)  eyes ;  when  it  comes  from 
the  father,  and  goes  to  a  daughter,  she  is  heterozygous 
for  bar  eye.  Baur  has  recently  recorded  the  appearance 
of  recessive  ( ?)  mutants  from  self -fertilized  plants  (snap- 
dragon) that  bred  true  at  once.  Punnett  has  described  n 
similar  case  (1919).  The  result  can  be  accounted  for,  if  a 
mutation  occurred  in  only  a  single  chromosome  far  enough 


MUTATION  251 

back  in  the  germ-tract  to  give  rise,  after  reduction,  botli 
to  pollen  and  to  ovules,  each  one  carrying  the  mutated 
genes.  Such  an  interpretation  is  supported  by  the  evi- 
dence from  Drosophila,  Avhere,  although  mutations  are 
much  more  numerous,  no  such  cases  have  been  observed, 
and  none  such  would  be  expected  if  mutation  occurs  in  a 
single  chromosome  at  a  time,  since  here  the  germ-cells 
come  from  separate  individuals. 

Probably  the  most  important  evidence  bearing  on  the 
nature  of  the  genes  is  that  derived  from  multiple  allelo- 
morphs. Now  that  the  proof  has  been  furnished  that  the 
phenomena  connected  with  these  cases  are  not  due  to  nests 
of  closely  linked  genes,  we  can  properly  appeal  to  these  as 
crucial  cases.  As  already  explained,  in  ever-increasing 
numbers  of  animals  and  plants,  series  of  genes  have  been 
found  in  each  of  which  mutant  characters  with  the  same 
normal  allelomorph  have  been  found.  These  mutant  char- 
acters of  each  series  are  also  allelomorphs  of  one  another 
— only  two  ever  existing  in  the  same  individual.  Ob- 
viously, not  all  such  mutants  can  be  due  to  the  absence 
of  a  factor  present  in  the  germ-plasm  of  the  wild  t>ije, 
since  only  one  kind  of  absence  is  thinkable.  If  to  save  the 
situation  for  the  theory  of  presence  and  absence  it  be 
assumed  that  only  a  part  of  the  original  gene  is  absent, 
and  a  different  part  in  each  case,  then  nothing  is  gained  by 
the  admission;  and  while  this  may  be  true  it  is  equally 
possible  that  the  genes  change  in  other  ways.  It  is  not 
essential  that  we  should  specify  the  nature  of  the  change, 
but  simpler  to  look  upon  the  mutant  gene  as  due  to 
some  kind  of  change  or  changes  that  have  taken  place 
in  the  original  germ-plasm  at  a  specific  locus — there  is 
nothing  known  at  present  to  furnish  even  a  clue  as  to  the 
nature  of  this  change. 

The  demonstration  that  multiple  allelomorphs  are 
modifications  of  the  same  locus  in  the  chromosome,  rather 
than  cases  of  closely  linked  genes,  can  come  only  where 
their  origin  is  known,  and  at  present  this  holds  only  in 


252 


PHYSICAL  BASIS  OF  HEREDITY 


the  case  (just  stated)  for  Indian  corn  and  for  the  fruit 
fly.  If  each  member  of  such  a  series  of  allelomorphs  has 
arisen  historically  from  the  preceding  one  in  the  series, 
by  a  mutation  in  a  locus  closely  associated  with  the  locus 
responsible  for  the  first,  they  would  be  expected  to  give 
the  wild  type  when  crossed;  and  as  the  proof  of  their 
allelomorphism  turns  on  the  failure  of  members  of  the 


B 


1   • 

2  • 

A 

3  • 

4  t 

5  t 

1   0 

1   • 

1   • 

1    • 

1    • 

2   • 

2   0 

2   • 

2   • 

2   • 

3   • 

3  • 

3   0 

3   • 

3  • 

4   • 

4  t 

4   • 

4   0 

4   • 

5  • 

5   • 

5  • 

5   • 

5   0 

a 

b 

c 

d 

e 

1  0 

a  0 

1    0 

1   0 

1   0 

2  t 

2  0 

2   0 

2  0 

2   0 

3  • 

3  • 

3   0 

3  0 

3   0 

4  • 

4  • 

4   • 

4   0 

4   0 

5  t 

5  • 

5   # 

5   • 

6   0 

Fig.  108. — Diagram  illustrating  mutation  in  a  nest  of  genes  so  closely  linked  that  no 

crossing  over' takes  place. 

series  to  show  the  atavistic  behavior  on  crossing,  it  is 
necessary,  as  stated,  to  know  how  they  arose.  This  may 
be  made  clear  by  the  following  illustration : 

Let  the  five  circles  of  Fig.  108,  A  represent  a  nest  of 
closely  linked  genes.  If  a  recessive  mutation  occurs  in 
the  first  one  (line  B,  a)  and  another  in  the  second  gene 
(line  B,  h),  the  two  mutants  a  and  h  if  crossed  should  give 
the  atavistic  type,  since  a  brings  in  the  normal  allelo- 
morph (B)  of  b,  and  h  that  (A)  of  a.  If  a  third  mutation 
should  occur  in  the  third  gene  it,  too,  will  give  the  atavistic 


MUTATION  253 

type  if  crossed  to  a  or  to  h.  Similarly  for  a  mutation  in 
the  fourth  and  in  the  fifth  normal  gene.  Now  this  is 
exactly  what  does  not  take  place  when  memhers  of  an 
allelomorphic  series  are  crossed — they  do  not  give  the 
wild  type,  but  one  of  the  other  mutant  types  or  an  inter- 
mediate character.  Evidently  independent  mutation  in  a 
nest  of  linked  normal  genes  will  not  explain  the  results 
if  the  new  genes  arise  directly  each  from  a  different  nor- 
mal allelomorph. 

But  suppose,  as  shown  in  Fig.  4  (line  C)  after  a  muta- 
tion had  occurred  in  the  first  gene  a  new  mutant,  h,  arose 
from  a  new  gene,  and  from  h  a  mutation  arose  in  a  third 
gene  c,  and  o  similarly  gave  rise  to  d;  then  a  crossed  to  h 
will  give  a  (or  something  intermediate  if  the  heterozygote 
is  an  intermediate  type).  Likewise  c  crossed  to  h  will 
give  h,  or  c  crossed  to  a  will  give  a,  etc.  If  mutant  allelo- 
morphic genes  in  a  series  such  as  6\  a,  h,  c,  d,  e,  arise  as 
successive  steps,  i.e.,  Ca  to  Cb  and  Cb  to  Cc,  etc.,  then 
the  hypothesis  of  closely  linked  genes  would  seem  to  be  a 
possible  interpretation  of  the  data,  but  if  they  do  not 
arise  in  this  way,  but  by  independent  mutations  from  the 
wild  type  (or  even  from  each  other,  but  not  seriatim),  then 
they  must  be  due  to  mutations  in  the  same  gene :  for,  to 
assume  that  they  are  not,  requires  that,  when  the  second 
mutation  took  place  both  gene  a  and  gene  b  mutated  at 
the  same  time,  and  that  when  c  appeared  three  genes 
mutated,  when  gene  d  appeared  four;  when  gene  e  five 
genes  mutated  at  once,  four  of  them  being  mutant  genes 
that  have  already  arisen  independently.  Such  an  inter- 
pretation is  excluded,  since  it  is  inconceivable,  even  in  a 
readily  mutating  form  like  Brosophila,  that  five  muta- 
tions could  have  occurred  at  the  same  time  in  distinct  but 
neighboring  loci.  As  has  been  stated,  the  evidence  from 
Brosophila  shows  positively  that  multiple  allelomorphs 
arise  at  random. 

Only  two  members  of  a  series  of  multiple  allelomorphs 
can  be  present  in  any  one  individual,  and  in  the  case  of 


254  PHYSICAL  BASIS  OF  HEREDITY 

genes  carried  by  the  sex-chromosome  only  one  can  exist 
at  a  time  in  the  sex  that  has  only  one  of  these  chromosomes. 
In  the  individual  with  two  mutant  allelomorphs  one  of 
them  replaces  the  normal  allelomorph  of  the  ordinary 
Mendelian  pair.  The  two  mutant  allelomorphs  behave 
towards  each  other  in  the  same  way  as  does  the  normal 
towards  its  mutant  allelomorphs.  It  is  doubtful  whether 
we  can  conclude  anything  more  from  this  relation  of  Men- 
delian pairs  than  we  knew  before/  although  there  is  at 
least  a  sentimental  satisfaction  in  knowing  that  both  nor- 
mal allelomorphs  can  be  replaced  by  mutant  ones  without 
altering  the  working  of  the  machinery. 

The  linkage  relation  of  each  member  of  a  series  of 
multiple  allelomorphs  to  all  other  genes  of  its  chromo- 
some is,  of  course,  the  same.  While  the  theory  of  identical 
loci  requires  this  as  a  primary  condition  it  is  not  legiti- 
mate to  use  this  evidence  as  a  proof  of  the  identity  of  the 
loci,  because  it  is  not  possible  to  work  with  sufficient  pre- 
cision in  locating  genes  by  their  relation  to  other  linked 
genes  to  distinguish  between  identical  loci  and  close- 
linked  genes. 

The  question  of  lethal  genes  has  attracted  in  recent 
years  increasing  attention,  both  on  account  of  their  fre- 
quency and  because  of  a  curious  complication  they  may 
produce  in  hiding  the  effects  of  other  genes  also  present. 
In  Drosophila  we  have  records  of  more  than  20  sex-linked 
lethals,  and  about  15  not  sex-linked,  and  scattering  records 
of  many  others.  Gametic  lethal  genes  are  those  that 
destroy  eggs  or  pollen  cells  that  contain  such  genes. 
Zygotic  lethal  genes  affect  the  embryo,  the  larva,  or  the 
adult,  so  that  it  dies.  In  the  case  of  the  garden  plant 
known  as  double  ^^ stocks,"  the  genetic  evidence  obtained 
by  Miss  Saunders  indicates  that  certain  kinds  of  pollen 
are  not  produced,  and  presumably  die  because  of  a  con- 
tained factor.    The  same  factor  does  not  kill  the  ovules, 

'  The  substitution  by  crossing  over  really  furnishes  as  good  a  demon- 
-^tration  of  this  point. 


MUTATION  255 

which  may  therefore  transmit  the  recessive  lethal  gene  to 
half  the  progeny.  How  far  the  frequent  occurrence  of  im- 
perfect pollen  grains  in  many  species  of  plants  is  due  to 
such  factors  is  still  uncertain. 

Belling  found  that  while  the  Florida  velvet  bean 
produces  normal  pollen  grains  and  ovules,  and  the  Lyon 
bean,  another  bean  of  the  same  genus,  also  produces  nor- 
mal gametes,  the  F^  hybrid  contains  50  per  cent,  abortive 
pollen  grains,  and  possibly  about  50  per  cent,  of  the  ovules 
are  abortive.  In  the  second  generation  (Fo)  half  of  the 
pollen  grains  of  half  of  the  plants  are  abortive.  The  other 
half  of  the  plants  have  normal  pollen  grains.  This  is  the 
result  expected  if  there  are  present  in  one  of  the  species 
the  factors  AAbh,  and  in  the  other  species  the  factors 
aaBB,  the  viable  gametes  in  the  F^  generation  being 
those  containing  Ab,  Ba,  and  the  two  gametes  that  die 
being  AB,  db. 

Other  observers  have  made  records  of  abortive  pollen 
in  hybrids,  but  without  knowing  the  condition  of  the 
pollen  in  the  parents  the  interpretation  of  the  results  is 
doubtful,  for,  as  Jeffrey  has  emphasized,  abortive  pollen 
is  a  characteristic  of  many  wild  species.  There  is  one 
fact  of  capital  importance  recorded  by  several  botanists, 
viz.,  that  the  degeneration  of  the  germ-ceUs  only  takes 
place  after  the  tetrad  has  been  produced,  and  only  in  some 
of  the  cells  of  each  tetrad.  In  other  words,  the  lethal 
effect  is  not  observed  until  the  chromosomes  have  under- 
gone reduction.  It  is  obvious  that  if  there  is  present  a 
recessive  lethal  for  the  germ-cells  (or  for  any  cells,  in 
fact),  it  causes  no  injury  in  the  presence  of  its  normal 
allelomorph,  but  kills  when  the  counter-effect  of  its  part- 
ner is  removed. 

Tischler  found  in  a  hybrid  currant  that  tetrad  fonna- 
tion  was  normal,  and  that  the  shrinking  of  the  pollen 
grains  occurred  afterwards.  Geerts  found  that  one-half 
of  the  pollen  grains  of  CEnothera  Lamarckiana  degen- 
erate, and  that  half  of  the  embryo  sacs  abort  in  the  tetrad 


256  PHYSICAL  BASIS  OF  HEREDITY 

stage.  Other  related  (wild)  species  and  genera  of  the 
evening  primrose  have  also  been  found  to  have  some 
abortive  pollen  and  ovules. 

Complete  or  nearly  complete  abortion  has  been  seen 
in  other  hybrids;  viz.,  by  Rosenberg  in  the  sundew,  by 
Osawa  in  the  Satsuma  orange,  by  Goodspeed  and  others 
in  the  hybrid  tobacco  {N.  tabacum  by  N.  sylvestris),  by 
Jesenko  in  the  wheat-rye  hybrid,  and  by  Sutton  in  the 
hybrid  between  the  Palestine  pea  {Pisum  humile)  and  the 
edible  pea.  These  cases  may  be  in  part  the  same  phe- 
nomenon and  in  part  a  different  one  comiected  with  fail- 
ure of  the  chromosome  to  conjugate  or  to  be  properly 
distributed  during  the  maturation  divisions. 

The  ^* yellow  mouse  case''  is  an  example  of  a  zygotic 
lethal  effect.  The  gene  that  produces  the  dominant  yellow 
color  is  lethal  in  double  dose,  so  that  all  homozygous  yel- 
low mice  die,  as  Cuenot  first  discovered,  and  as  has  been 
more  positively  demonstrated  by  the  work  of  Castle  and 
Little.  There  is  some  evidence  indicating  that  these  homo- 
zygotes  die  as  young  embryos.  Little  has  also  shown  that 
black-eyed  white  mice  carry  a  lethal,  that  acts  in  the  same 
way.  In  Drosophila  there  is  a  sex-linked  recessive  lethal 
factor  that  causes  the  development  of  tumors  in  the  larvae, 
destroying  every  male  larva  that  contains  the  sex-chromo- 
some carrying  this  gene.  This  effect,  discovered  by 
Bridges,  has  been  the  basis  for  an  extensive  series  of 
experiments  by  Miss  Stark.  The  gene  is  present  in  the 
X-chromosomes ;  it  follows  the  rules  for  all  sex-linked 
genes  in  its  inheritance.  The  females  of  the  stock  are  of 
two  kinds :  One  has  the  lethal  in  one  sex-chromosome,  and 
its  normal,  dominant  allelomorph  in  the  other.  Such  a 
female  has  survived  because  the  effect  of  the  lethal  gene 
is  counteracted  by  the  effect  of  its  normal  allelomorph. 
Half  of  her  sons  get  the  affected  chromosome.  All  such 
sons  develop  the  tumor — one  or  more  melanitic  growths 
that  appear  in  the  imaginal  discs  or  in  other  parts  of  the 
larva.    The  other  sons  get  the  other  chromosome  with  the 


MUTATION 


257 


normal  allelomorph.  They  never  produce  a  tumor  and 
never  transmit  the  disease.  The  same  mother  that  gave 
these  two  kinds  of  sons — having  been  fertilized  by  a  nor- 
mal male,  since  no  affected  males  exist— produces  also  two 
kinds  of  daughters,  one  containing  the  gene  for  the  tumor 
(and  its  normal  allelomorph),  the  other  having  two  nor- 
mal genes.  The  former  transmit  the  disease  as  just 
explained,  the  latter  daughters  are  perfectly  normal  and 
do  not  transmit  the  disease. 

Other  lethal  genes  kill  the  pupae,  a  few  of  them  even 
allow  the  fly  occasionally  to  come  through,  but  such  flies 
rarely  propagate.  Certain  races  of  Drosophila  have  ster- 
ile or  nearly  sterile  females,  other  races  sterile  males. 
The  sterility  is  here  lethal  in  so  far  as  it  affects  the  germ- 
cells.  Some  effects  on  other  characters  are  also  generally 
to  be  seen. 

The  presence  of  a  lethal  gene  near  to,  i.e.,  linked  to, 
another  mutant  gene  may  affect  the  kinds  of  individ- 
uals that  appear  because  owing  to  the  linkage  the  other 
mutant  character  fails  to  appear,  except  when  crossing 
over  takes  place.  Some  examples  of  this  relation  may  be 
given.  There  is  a  mutant  race  called  beaded  (Fig.  109) 
in  which  the  margin  of  the  wing  is  irregularly  broken, 
giving  the  appearance  of  a  beaded  edge.  The  gene  for 
beaded  is  dominant,  and  lethal  when  homozygous. 

As  in  the  case  of  the  yellow  mouse,  only  the  hybrid 
(heterozygous)  combination  exists,  and  consequently 
when  two  beaded  flies  mate  they  produce  two  beaded  to 
on^  normal  fly,  as  shown  in  Fig.  110.  Here  the  first  pair 
of  vertical  lines  stand  for  the  pair  of  third  chromosomes 
present  in  the  egg  before  its  reduction.  The  two  genes 
here  involved,  that  for  beaded  and  its  allelomorph  for 
normal,  are  indicated  at  the  lower  end  of  the  vertical  lines. 
The  two  corresponding  chromosomes  in  the  male  are 
represented  to  the  right  of  the  last.  After  the  ripening  of 
the  germ-cells  each  egg  and  each  sperm  carries  one  or 
the  other  of  these  chromosomes.  Chance  meetings  of  egg 
17 


258 


PHYSICAL  BASIS  OF  HEREDITY 


and  sperm  are  indicated  in  the  figure  by  the  arrow-scheme 
below,  which  gives  the  combinations  (classes)  included  in 
the  four  squares.  The  double  dominant  BB  is  the  class 
that  does  not  come  through.  The  result  is  two  beaded 
(heterozygous)  to  one  normal  fly. 

The  beaded  stock  remained  in  this  condition  for  a  long 
time ;  although  selected  in  every  generation  for  beaded,  it 
did  not  improve,  but  continued  to  throw  33  per  cent,  of 
normal  flies.    Then  it  changed  and  bred  nearly  true. 


Eggs 


Sperm 


B-- 


t 


N 


B"-     --N 


5  ^^ 

ixt 

B  N. 


Beaded   :   1  Normal, 


Fig.  110. — Diagram  showing  the  relation  of  the  chromosomes  (represented  by  the 
vertical  rods)  in  a  cross  of  "beaded"  by  "beaded."  Flies  homozygous  for  beaded  die  as 
indicated  by  the  cross-hatched  square. 

The  change  must  have  been  due  to  the  appearance  of 
another  lethal  factor  (now  called  lethal  three,  here  Zi)  in 
Fig.  111).  Such  a  gene  was  found  in  the  race  when 
studied  later  by  Muller. 

The  lethal  gene  that  appeared  in  the  beaded  stock  was 
also  in  the  third  chromosome,  and  in  the  chromosome  that 
is  the  mate  of  the  one  carrying  the  gene  for  beaded,  i.e., 
in  the  normal  third  chromosome  of  the  beaded  stock.  The 
lethal  gene  lies  so  near  to  the  level  of  the  beaded-normal 


t-H 

Q 


O 
CO 


o 

a 


o 

Co 


^ 


a 


0 


1<J-.- 


1^-,.. 


MUTATION 


259 


pair  of  genes  that  almost  no  crossing  over  takes  place 
between  the  levels  occupied  by  the  two  pairs.  These  rela- 
tions are  illustrated  in  the  next  diagram,  Fig.  111.  Here 
again  the  two  pairs  of  vertical  lines  to  the  left  represent 
the  two  third-chromosome  pairs  in  the  female  and  to  the 
right  in  the  male.  The  location  of  the  two  pairs  of  genes 
involved,  N-l^  and  B-N,  are  indicated.  These  combina- 
tions give  the  four  classes  in  the  squares  of  which  two 
classes  die,  vi^.,  NNBB   (pure  for  beaded)  and  l,l,NN 


Eggs 


Sperm 


N  -- 

B  -- 


.-1,  N 
"N   B 


NB 

IN 

1 

^ 

"^^y^ 

All  Beaded 


KB 


NB 


liN 


liH 


Fig.  111. — Diagram  to  show  how  the  appearance  of  a  lethal  near  beaded  rausre 
the  stock  to  produce  only  beaded  except  for  the  small  number  of  crossovers,  as  shown 
by  the  next  diagram. 

(pure  for  lethal  three).  The  result  is  that  only  beaded 
flies  come  through,  and  since  all  these  are  heterozygous 
both  for  B  and  l-^,  the  process  is  self -perpetuating. 

If  the  preceding  account  represented  all  of  the  facts  in 
the  case,  the  stock  of  beaded  should  have  bred  perfectly 
true,  but  it  has  been  shown  in  Brosoplula  that  crossing 
over  between  the  members  of  the  pairs  of  genes  takes 
place  in  the  female.  Hence  we  should  expect  a  complica- 
tion due  to  crossing  over  here  unless  the  level  of  the  two 
pairs  of  genes  was  so  nearly  the  same  as  to  preclude  this 
possibility.     In  fact,  in  addition  to  the  beaded  flies  the 


260 


PHYSICAL  BASIS  OF  HEREDITY 


stock  in  this  condition  alone  should  give  10  per  cent,  of 
crossing  over, i.e., it  should  still  produce  a  small  percentage 
of  normal  flies.  It  so  happened,  however,  that  there  was 
present  in  the  stock  a  third  gene  that  lowers  the  amount 
of  crossing  over  in  the  female  to  such  an  extent  that,  for 
the  two  ' '  distances ' '  here  involved,  practically  none  takes 
place.  When  it  does,  a  normal  fly  appears,  but  this  is 
so  seldom  that  such  an  occurrence,  if  it  happened  in  a 
domesticated  form  of  which  the  wild  type  was  unknown. 


Crossover 
Eggs 


Sperm 


H-- 


-1,   N-.   -li 
--B   B--   --N 


NB 

IN 

'i^ 

^y^^ 

^:^ 

IBead . INorm . 0 • 5^ 


NN 


I 


KB 


liB 


liH 


Fig.   112. — Diagram  showing  the  results  of  crossing  over  in  a  stock  containing  both  beaded 

and  lethal,  as  shown  in  Fig.  111. 

would  be  set  down  as  a  mutation  like  that  shown  by  the 
evening  primrose. 

The  third  factor  that  entered  into  the  result  is  not 
unique,  for  Sturtevant  has  shown  that  crossover  factors 
are  not  uncommon  in  Drosophila.  The  analysis  that  Mul- 
ler  has  given  for  beaded,  while  theoretical,  is  backed  up 
by  the  same  kind  of  genetic  evidence  that  is  accepted  in 
all  Mendelian  work.  It  makes  an  assumption  but  one  that 
can  be  demonstrated  by  any  one  who  will  make  the  neces- 
sary tests.  It  is  also  possible  to  produce  at  will  other  bal- 
anced lethal  stocks  that  will  ''mutate'^  in  the  sense  that 


MUTATION 


2ni 


they  will  throw  off  a  small  predictable  number  of  a 
''mutant"  type— a  type  that  we  can  introduce  into  the 
stock  for  the  express  purpose  of  recovering  it  by  such  an 
apparent  mutation  process. 

For  example,  dichete  is  a  third  chromosome  dominant 
wing-and-bristle  character  and,  like  beaded,  a  recessive 
lethal.  Sturtevant  bred  flies  with  the  gene  for  dichete 
in  one  of  the  third  chromosomes  and  with  a  gene  for  the 
recessive  eye-color,  peach,  in  the  other  for  several  genera- 


Hon  crossover  eggs  «^^^^ 
(95^  of  total)  ^P®^"" 


N 

Ni- 


N 
la 

P 


D'NN 
Dichete 

NAP 

Dm 

Dichete 

^y^    ^ 
^^M^-^ 

DNN  Hlap 

tXI 

d'hk  Nis  p 


Fig.  113. — Diagram  illustrating  how  in  the  presence  of  a  dominant  factor,  dichete,  and 
a  lethal  in  its  homologous  chromosome  at  about  the  same  level,  together  with  another 
factor,  peach-colored  eyes  (p),  gives  the  result  shown  in  the  squares.  No  peach  appears 
in  the  offspring  except  where  crossing  over  takes  place  as  shown  in  the  next  diagram. 

tions.  A  lethal  appeared  by  mutation  in  the  peach-bearing 
chromosome  very  near  the  level  of  the  dichete  gene  in 
the  opposite  chromosome. 

The  order  of  these  genes  is  shown  in  Fig.  113.  This  is 
then  a  balanced  lethal  stock  that  throws  only  dichete 
flies,2  except  for  a  small  percentage  of  dichete  peach  flies 
due  to  crossing  over.  The  result  for  the  non-crossover 
classes  is  shown  in  the  square  to  the  right.  Only  two 
of  the  four  classes  come  through :  the  two  that  die  are  the 

"Very  rarely  a  crossover  not-dichete  fly  will  appear. 


262 


PHYSICAL  BASIS  OF  HEREDITY 


one  pure  for  dichete  and  the  one  pure  for  lethal.  The  sur- 
viving classes  continue  to  produce  the  same  kind  of 
offspring  since  they  are,  like  the  parents,  heterozygous 
for  the  two  lethal  factors.  But  the  factors  are  not  near 
enough  together  to  prevent  crossing  over,  which  occurs 
in  about  5  per  cent,  of  cases  between  the  lethal  and  peach 
genes.  The  next  diagram,  Fig.  114,  shows  how  when 
crossing  over  takes  place  in  the  female,  there  result  four 


Crossover  eggs   orx^v,™ 
(5f«  of  total)  ^P^^°^ 


N-- 
P" 


-N 

D'- 

-1» 

N- 

-N 

N- 

CfNp 
Dich. Peach 

Dichet© 

y^^^^y^ 

95+95+5  Dichete 
5  Dichete  Peach 


=97  y^ 


2^2 


1X1 

D'NN  NI2  p 

Fig.   114. — Diagram  illustrating  crossing  over  of  factors  shown  in  Fig.   113. 

classes  (see  squares),  of  which  two  die  (as  before),  and 
of  the  two  that  survive  one  is  dichete  peach.  Taking  both 
non-crossover  and  crossover  results  together,  the  expec- 
tation is  95  +  95  -f  5  dichete  to  5  dichete  peach  or  97% 
to  21/2-  This  stock  then  breeds  true  for  dichete  without 
showing  the  gene  it  carries  for  peach  eye-color  except  in 
a  small  percentage  of  cases,  and  if  the  peach-eyed  fly 
should  be  unable  to  establish  itself  in  nature,  like  some 
of  the  (EnoDiera  mutants,  the  stock  would  not  be  changed 
by  it,  but  continue  to  throw  off  a  few  ''mutants^'  with 
peach-colored  eyes. 


MUTATION 


263 


Now  this  process  is  not  what  is  ordinarily  meant  by 
mutation,  for  we  mean  by  the  latter  that  a  new  type  has 
suddenly  arisen  in  the  sense  that  some  change  has  taken 
place  in  the  germ-plasm— a  new  gene  has  been  formed. 
The  process  here  described  is  one  of  recombination  of 
genes  shown  by  Mendelian  hybrids,  the  only  unusual  fea- 
ture being  that  all  the  phenomena  involved  do  not  come  to 
the  surface  because  many  classes  are  destroyed  by  lethals. 

The  results  are  interesting  also  in  another  way.  It  has 
been  assumed  by  those  who  think  that  0.  Lamarckiana  is 


Fig.   115. — Rosettes  of  the  twin  hybrids  of  the  evening  primrose,  the  plant  to  the  left  is 
called  laeta,  and  that  to  the  right  velutina.     (After  De  Vries.) 

a  hybrid  that  the  mutant  types  are  only  the  segregation 
products  of  the  types  or  combinations  that  went  in  to  pro- 
duce the  hybrid.  But  the  DrosopJiila  cases  show  that 
balanced  lethal  stocks  may  arise  within  stocks  themselves 
by  the  appearance  in  them  of  lethal  factors  closely  linked 
to  other  factors — new  or  old  ones.  Wlien  new  genes  arise 
in  such  lethal  stocks  the  process  may  be  one  of  tnie  muta- 
tion, but  the  revelation  of  the  presence  of  the  gene  is 
hindered  by  the  lethal  factors,  so  that  when  the  clujracter 
appears,  it  appears  as  a  *^new'^  mutant,  but  is  in  reality 
due  to  recombination  of  mutant  genes  that  had  arisen  in 
an  earlier  generation.     As  a  matter  of  fact,  the  first 


264 


PHYSICAL  BASIS  OF  HEEEDITY 


appearance  of  even  ordinary  mutants,  unless  they  be 
dominant,  must  come  two  or  more  generations  after  the 
mutation  has  taken  place ;  for,  the  evidence  indicates  that 
mutation  appears  in  only  one  chromosome  at  a  time.^ 
In  the  case  of  sex-linked  genes,  however,  any  mutation 
that  takes  place  in  one  of  the  X-chromosomes  of  the  mother 
is  revealed  if  the  egg  containing  it  gives  rise  to  a  son, 
because  he  has  but  one  X-chromosome  and  that  comes 
from  his  mother. 

The  delayed  occurrence  then  of  mutants  in  balanced 
stocks  is  not  different  from  the  delay  in  other  stocks — 


v7lld 


BesuSed 


»+      +H       K-l- 
N 


--«  «--  --Xj 

•      +H       B+      -f-H 


1X1 

HB  1,N 


li 


Twin  Hybrids, 


Fig.  116. — Diagram  illustrating  balanced  lethals  and  twin  hybrids. 


only  when  the  recombinations  occur  in  balanced  lethal 
stocks  they  must  have  been  preceded  by  crossing  over, 
which  diminishes  the  number  of  mutants  that  appears. 
The  number  of  mutants  that  appears  is  determined  by 
the  distance  of  the  genes  for  the  character  from  the 
nearest  lethal  gene. 

One  of  the  most  interesting  features  of  Lamarck's 
primrose  arises  when  it  is  bred  to  certain  other  species 
or  varieties.  It  gives  rise  to  two  kinds  of  offspring 
called  Twin  Hybrids,  to  which  De  Vries  gives  the  names 
Iceta   and    velutma    (Fig.    115).     Now   it   is    a   feature 

'  If  in  self-fertilizing  forms  a  mutation  takes  place  so  early  in  the 
germ-plasm  that  it  gets  into  both  eggs  and  sperm  the  new  character  may 
appear  at  once    (see  ante). 


MUTATION 


265 


of  balanced  lethal  stocks  like  beaded  that  they  repeat 
precisely  this  phenomenon.  For  instance,  if  a  beaded 
male  is  crossed  to  wild  female,  two  kinds  of  offsprini^  are 
produced,  vi^.,  beaded  and  normal.  A  similar  process 
would  account  for  twin  hybrids  in  CEnothera  crosses. 
There  is  another  peculiar  phenomenon  that  has  been 
described  for  crosses  in  the  evening  primroses,  viz.,  the 
occurrence  in  F^  of  four  types.  This  phenomenon,  too, 
can  be  imitated  in  Drosophila  by  crossing  balanced  lethal 
dichete  to  balanced  lethal  beaded  (Fig.  117). 


Diohet© 

2 


Beaded 


IT. 


+  N 

l2 


N 

N 


S        N    -      -li 


N 


--H 

■-N 


B+       -N 


D  N  K  N 

N  N  N  B 
Bead.Dichete 


N  H  N  B 
Beaded 


D  N  N  N 
N  N  liM 
Cichete 


N  N   li  N 
Kormal 


n  t 

Both  stocks  breed  true.         Four  types  in  P^    1:1: Itl 

Fig.  117. — Diagram  illustrating  lethals  and  four  types  in  Fi. 

Other  parallels  might  be  cited,  but  these,  I  think,  will 
suffice  to  indicate  that  the  discovery  of  balanced  lethal 
stocks  may  solve  some  at  least  of  the  outstanding  difficul- 
ties of  mutation  and  inheritance  in  (Enothera,  and  bring 
it  into  line  with  other  groups.  There  are,  of  course,  other 
peculiarities  of  the  evening  primrose  that  such  zygotic 
lethals  will  not  explain;  such,  for  instance,  as  the  15-chro- 
mosome  type,  and  0.  gigas.  But  these  cases  are  already 
on  the  road  to  solution. 

The  occurrence  of  other  lethals,  called  gametic  lethals, 


266  PHYSICAL  BASIS  OF  HEEEDITY 

that  kill  the  germ-cells — gametes — before  they  are  ready 
for  fertilization,  has  already  been  invoked  by  De  Vries 
and  others  to  explain  the  peculiarity  of  '^double  recipro- 
cal hybrids.'' 

Is  THE  Direction  of  Mutation  Given  in  the  Constitution 

OF  THE  Genes? 

When  writers  have  brought  forward  evidence  of  con- 
tinuous and  progressive  change  in  a  character,  they  have 
not  concerned  themselves  with  the  analysis  of  the  change 
in  the  germ-plasm  that  has  brought  it  about — in  fact,  in 
most  of  these  cases  the  possibility  of  advance  in  a  princi- 
pal gene  or  of  advance  through  modifying  genes  has  not 
been  appreciated  or  even  understood.  Paleontologists 
who  have  in  the  main  been  the  strong  advocates  of  ortho- 
genesis have  based  their  conclusions  on  the  observed 
advances  in  a  character  in  the  same  series  and  in  ^^  paral- 
lel'' series.  They  overlook  the  fact  that  to-day  there  is 
experimental  evidence  demonstrating  that  variations  as 
small  even  as  those  they  record  have  been  shown  to  rest 
on  mutational  stages.  If  the  progress  has  been  in  the 
direction  of  adaptation,  natural  selection  of  small  mutant 
differences  will  completely  cover  their  findings.  If  it  is 
claimed  that  in  some  of  these  cases  the  orthogenetic  series 
is  not  in  the  line  of  adaptive  advance,  the  burden  of  proof 
lies  heavily  on  their  shoulders.  Moreover,  the  fact,  that 
recent  work  has  made  clear,  that  genes  generally  have 
more  than  a  single  effect  on  the  organization,  opens  wide 
the  door  of  suspicion,  for  the  observed  morphological 
progress  might  be  a  by-product  of  influences  that  have 
other  and  important,  though  unseen  or  unknoAvn,  effects. 
In  a  word,  an  orthogenetic  series  of  changes  does  not  in 
itself  without  a  closer  analysis  than  has  as  yet  been  fur- 
nished, establish  that  an  innate  principle,  urge,  vis-a-tergo, 
''kick/'  or  vital  ''force''  is  causing  the  successive  moves. 
The  genetic  evidence  concerning  multiple  factors  must 
create  at  least  a  strong  suspicion  against  the  '^will  to 


MUTATION  267 

believe''  in  the  mystic  sentiments  for  which  these  terms 
always  stand.  That  a  progressive  series  of  advances  in  a 
gene  might  take  place  with  a  consequent  advance  in  the 
many  characters  involved  is  thinkable,  especially  if  it  could 
be  shown  that  environmental  changes  cause  "parallel  prog- 
ress in  the  gene,  and  this  in  turn  on  the  character.  How 
probable  this  is  the  reader  must  decide  for  himself  in  the 
light  of  the  very  clear  evidence  that  each  character  is 
affected  by  changes  in  many  genes  differently  located  in 
the  germ-plasm,  and  that  it  is  not  a  progressive  change  in 
one  gene  that  makes  selection  possible,  but  changes  in  any 
one  of  many  genes. 

Chance  Mutation  and  Natural  Selection 
The  mutation  process  rests  its  argument  for  evolution 
on  the  view  that  among  the  possible  changes  in  the  genes, 
some  combinations  may  happen  to  produce  characters  that 
are  better  suited  to  some  place  in  the  external  world  than 
were  the  original  characters.  Apparently  this  appeal  to 
chance,  like  Darwin's  appeal,  has  offended  some  of  the 
adherents  of  the  doctrine  of  organic  evolution,  because  it 
has  seemed  to  them  inconceivable  that  chance  could  ever 
bring  about  the  assembling  of  such  an  intricate  piece  of 
machinery  as  a  highly  complex  organism.  The  attempt 
to  mitigate  the  rude  shock  of  the  appeal  to  chance  was 
made  by  Darwin  by  pointing  out  that  evolution  had  been 
gradual  and  that  the  assemblage  has  not  taken  place  out 
of  chaos,  but  each  stage  has  been  built  up  on  one  a  little 
less  complex  than  the  preceding  one.  Nevertheless  the 
fact  remains  that  persistent  efforts  continue  to  be  made 
from  time  to  time  to  introduce  into  the  theory  of  evolution 
some  sort  of  directive  mystical  agency.  The  Laniarckiaii 
theory  has  tried  to  bring  about  a  more  immediate  relation 
between  the  organism  and  its  environment  of  such  a  kind 
that  the  adaptive  change  that  appears  in  the  body  as  a 
result  of  a  reaction  between  the  environment  and  the  ani- 
mal or  plant,  is  reflected  into  the  germ-plasm.     Bergson 


268  PHYSICAL  BASIS  OF  HEEEDITY 

has  cut  the  knot  by  postulating  an  innate  adaptive  respon- 
siveness of  the  animal  to  every  critical  situation  that  calls 
out  a  response.  The  adherents  of  orthogenesis  appeal, 
apparently — in  so  far  as  they  commit  themselves — to 
some  sort  of  innate  principle  that  causes  advance  in  com- 
plexity along  one  line,  and  they  seem  to  hint  at  times  even 
along  directed  lines  of  adaptation.  Still  more  elusive  are 
vague  appeals  made  to  some  unknoAvn  principle — some 
sort  of  mysterious  element,  some  ^'Bion/'  resident  in 
living  material  and  peculiar  to  it  that  is  responsible 
for  evolution. 

We  are  not  concerned  with  any  of  these  so-called 
agents,  but  there  is  a  relation  between  chance  and  evolu- 
tion shown  by  living  things  that  has  been  largely  neglected, 
or  at  least  vaguely  referred  to,  even  by  natural  selection- 
ists, that  is  of  fundamental  importance  when  evolution  is 
treated  as  a  phenomenon  of  chance. 

This  relation  may  be  stated  in  a  general  way  as  fol- 
lows :  Starting  at  any  stage,  the  degree  of  development  of 
any  character  increases  the  probability  of  further  stages 
in  the  same  direction.  The  relation  can  better  be  illus- 
trated by  specific  cases.  The  familiar  example  of  tossing 
pennies  will  serve.  If  I  have  thrown  heads  five  times  in 
succession,  the  chance  that  at  the  next  toss  of  a  penny  I 
may  make  a  run  to  six  heads  is  greater  than  if  I  tossed 
six  pennies  at  once.  Not,  of  course,  because  five  separate 
tosses  of  heads  will  increase  the  likelihood  that  at  the 
next  toss  a  head  rather  than  a  tail  will  turn  up,  but  only 
that  the  chances  are  equal  for  a  head  or  a  tail,  so  that  I 
have  equal  chances  of  increasing  the  run  to  six  by  that 
throw,  while  if  I  tossed  six  pennies  at  once  the  chances 
of  getting  six  heads  in  one  throw  are  only  once  in  64  times. 

Similar  illustrations  in  the  case  of  animals  and  plants 
bring  out  the  same  point.  If  a  race  of  men  average  5  feet, 
10  inches,  and  on  the  average  mutations  are  not  more  than 
two  inches  above  or  below  the  racial  average,  the  chance 
of  a  mutant  individual  appearing  that  is  6  feet  tall  is 
greater  than  in  a  race  of  5-foot  men.    If  increase  in  height 


MUTATION  269 

is  an  advantage,  the  taller  race  has  a  better  chance  than 
the  smaller  one.  This  statement  does  not  exclude  the 
possibility  that  a  short  race  might  happen  to  beat  out 
in  height  a  taller  race,  for  it  might  more  often  mutate ; 
but  chance  favors  the  tall.  In  this  sense  evolution  is  more 
likely  to  take  place  along  lines  already  followed,  if  further 
advantage  is  to  be  found  in  that  direction. 

A  rolling  snowball  that  already  weighs  10  pounds  is 
more  likely  to  reach  15  pounds  than  is  another  that  has 
just  begun  to  roll.  The  chance  that  a  monkey  could  change 
into  a  man  is  far  greater  than  that  an  amoeba  could 
make  the  transition.  The  monkey  has  accumulated,  so 
to  speak,  so  many  of  the  things  that  go  to  make  up  a 
man  that  his  chance  of  reaching  that  goal  is  vastly  greater 
than  the  amoeba's. 

There  is  also  a  peculiarity  of  animals  and  plants  that 
assists  greatly  towards  progress  along  lines  already 
started.  The  individual  multiplies  itself,  and  a  new 
mutant  character  that  is  advantageous  becomes  estab- 
lished in  a  large  number  of  individuals,  or  even  in  all  indi- 
viduals of  the  race.  The  number  of  individuals  increases 
the  chance  of  a  new  random  mutation  along  the  path 
already  taken.  It  is  true  that  the  chance  of  a  random 
variation  in  the  opposite  direction  is  equally  great,  but 
as  this,  by  hypothesis,  is  the  less  advantageous  direction 
it  will  fail  to  establish  itself  in  numbers. 

Darwin  built  up  his  evidence  for  natural  selection  and 
even  for  evolution,  on  the  artificial  selection  of  variations 
of  animals  and  plants  under  domestication.  It  is  in  this 
field  that  the  student  of  Mendelism  revels.  Almost  without 
exception  he  finds  that  the  domestic  races  of  animals  and 
plants  are  built  up  by  mutational  differences.  It  is  this 
evidence  that  to-day  is  a  hundredfold  stronger  for  the 
theory  of  evolution  than  it  was  in  Dar\vin  's  time. 

The  slightest  familiarity  with  wild  species  will  suffice 
to  convince  any  one  that  they  differ  from  each  other 
generally,  not  by  a  single  Mendelian  difference,  but  by 


270  PHYSICAL  BASIS  OF  PIEEEDITY 

a  number  of  small  differences.  The  student  of  Men- 
delian  heredity  at  least  is  not  likely  to  fall  into  the  error 
of  identifying  single  Mendelian  dilferenees  with  the  sum 
total  of  differences  by  which  wild  types  and  often  even 
wild  varieties  differ  from  each  other,  but  whenever  he 
has  had  an  opportunity  to  study  these  single  dilfer- 
enees in  wild  varieties  he  has  found  that  they  seem  to 
originate  and  to  be  inherited  in  the  same  way  as  other 
Mendelian  characters. 

Species  as  G-roups  of  Genes 

If  related  species  have  many  genes  in  common  they 
may  be  expected  to  produce  at  times  the  same  mutants. 
In  fact,  it  is  not  at  all  uncommon  to  find  even  in  Men- 
delian literature  such  forms  as  albinos  spoken  of  as  though 
they  represent  the  same  mutation  wherever  it  arises. 
Attractive  as  such  a  view  appears,  experience  has  shown 
that  it  is  very  unsafe  to  judge  as  to  the  nature  of  the  muta- 
tion from  the  appearance  of  the  character  alone.  Two 
different  white-flowered  races  of  sweet  peas  are  known 
which  give  the  wild  purple-flowering  pea  when  crossed, 
showing  that  they  represent  different  mutations.  Simi- 
larly, at  least  two  recessive  white  races  of  fowls  are 
known,  as  well  as  a  third  dominant  white  race.  Three 
independent  mutations  have  produced  white  birds. 
Whether  albino  mice,  rats,  rabbits,  squirrels  and  guinea 
pigs  have  arisen  through  a  mutation  in  a  common  gene 
cannot  be  determined  because  they  cannot  be  crossed 
to  each  other.  When  we  consider  that  many  factors  may 
combine  to  produce  a  given  pigTQented  animal,  and  that  a 
change  in  any  one  of  them  may  affect  the  end  result,  it 
will  be  evident  that  the  expectation  would  be  against 
rather  than  for  the  conclusion  that  the  same  gene  had 
changed  in  all  cases.  Only  when  it  could  be  shown  that  a 
particular  gene  of  the  complex  is  more  likely  to  change 
in  a  given  direction  than  other  genes  of  the  complex  would 
this  interpretation  become  plausible. 


1 


MUTATION  271 

There  is  evidence  in  Drosophila  melanogaster  show- 
ing that  the  same  mutation  to  white  eyes  has  occurred  sev- 
eral times,  and  the  additional  and  all-important  proof 
has  been  obtained  that  it  is  the  same  locus  that  has  pro- 
duced the  white-eyed  mutant.  This  may  appear  to  give 
some  slight  support  to  the  view  that  albino  mutants 
appearing  in  other  related  species  may  be  due  to  the  same 
mutative  changes,  but  without  additional  evidence  this 
conclusion  is  problematical. 

In  the  mammals  melanic  individuals  have  been  fre- 
quently described,  but  there  is  no  direct  evidence  to  show 
that  they  are  due  aU  to  the  same  change.  In  the  roof  rat 
there  is  a  black  type  that  is  dominant  to  the  gray  of  this 
race,  while  the  black  type  of  the  Norway  rat  is  recessive 
to  the  gray  of  that  race.  It  seems  probable  that  they  are 
different  mutations,  but  not  necessarily  so. 

Yellow  in  the  mouse  is  dominant  and  lethal ;  two  races 
of  yellow  rats  are  known,  both  recessive  forms.  The  rela- 
tion of  yellow  to  black  in  mice  is  different  from  the  rela- 
tion of  either  of  the  yellows  to  black  in  the  Norway  rat. 
If  the  blacks  are  the  same  mutant  the  yellows  are  differ- 
ent ;  if  either  yellow  of  the  rat  is  the  same  as  the  yellow  of 
the  mouse,  the  blacks  must  be  different,  etc. 

The  uncertainty  of  reaching  any  conclusion  in  regard 
to  the  nature  of  the  mutation  from  the  appearance  of  the 
character  of  the  mutant  is  excellently  illustrated  in  such 
a  group  of  mutants  as  that  of  the  fruit  fly,  where  a  con- 
siderable number  of  cases  are  known  in  which  mutants 
that  are  almost  indistinguishable  externally  have  been 
shown  to  be  due  to  mutations  in  different  parts  of  the 
germ-plasm.  There  are  five  kinds  of  black  mutants,  three 
or  more  yellows  and  several  eye  colors  that  are  practically 
indistinguishable.  The  evidence  showing  their  difference 
is  obtained  from  the  results  of  crossing,  where,  as  a  rule 
(except,  for  example,  cases  of  complete  or  incomplete 
dominants),  reversion  to  the  wild  type  occurs.    In  addi- 


272  PHYSICAL  BASIS  OF  HEREDITY 

tion,  the  localization  of  the  gene  causing  the  modification 
shows  them  to  be  different. 

The  method  of  localizing  genes  offers  an  opportun- 
ity for  obtaining  evidence  in  regard  to  like-mutants  in 
related  species  that  cannot  be  crossed,  and  a  step  forward 
in  this  direction  has  been  taken  by  C  W.  Metz  for  other 
species  of  the  genus  DrosophUa.  In  one  species,  D.  virilis, 
he  has  found  12  mutants,  and  these  fall  into  three 
groups  of  linked  genes.  Three  of  them,  yellow,  forked 
and  confluent,  resemble  externally  characters  of  D. 
melanog aster.  Yellow  and  forked  are  sex-linked  and  look 
like  the  same  characters  in  melanog  aster.  Confluent  is 
like  a  second  chromosome  character  of  the  same  name  in 
melanog  aster  in  three  respects :  first,  in  that  the  structures 
are  similar ;  second,  in  that  the  character  is  dominant  in 
both  forms;  and,  third,  in  that  it  is  lethal  in  the  homo- 
zygous state.  The  terminal  position  of  yellow  and  the 
large  amount  of  crossing  over  with  forked  are,  roughly 
speaking,  the  same  in  both. 

Even  in  this  case  further  work  is  needed,  first,  because 
within  the  same  species  the  occurrence  of  similar-looking 
characters  due  to  different  factors  is  known,  e.g.,  there 
are  two  genes  for  yellow  color  (yellow  and  lemon)  in  the 
first  chromosome  of  D.  melanogaster  and  in  the  same  part 
of  that  chromosome,  and  second,  because  it  is  not  to  be 
expected  that  the  number  of  crossovers  would  be  identi- 
cally the  same  between  the  same  loci  in  different  species, 
since  marked  variations  are  known  within  a  single  species. 
Unless  such  species  can  be  crossed,  the  only  convincing 
evidence  that  we  can  hope  to  get  will  be  to  establish 
the  sa77ie  linear  order  in  the  chromosome  for  several 
genes  whose  characters  appear  to  be  the  same  or  similar. 

Other  evidence  of  a  different  kind  also  helps  to  make 
probable  that  the  same  mutations  occur  in  different 
spei^ies.  For  example,  in  cases  where  a  mutant  gene  pro- 
duces a  number  of  changes  in  different  parts  of  the  body, 
the  probability  that  it  is  the  same  as  one  in  a  different 
si)ecies  that  causes  the  same  modifications,  is  in  propor- 


MUTATION  273 

tion  to  the  number  of  the  same  kinds  of  change  that  they 
produce.  The  two  following  cases  recorded  by  Sturtevant 
illustrate  this  relation: 

Two  species,  viz.,  Drosophila  melcmogaster  and  D. 
funehris,  have  each  produced  a  mutation  called  notch. 
This  character,  notch,  involves  not  only  a  notching  at  the 
end  of  the  wings  but  also  the  thickening  of  the  second  and 
fifth  veins  of  the  wings,  frequent  reduction  and  roughen- 
ing of  the  eyes,  inequalities  of  the  rows  of  hairs  on  the 
thorax,  frequent  doubling  of  the  anterior  scutellar  bristles, 
and  a  recessive  lethal  effect.  The  character  is  also  dom- 
inant and  sex-linked.  It  is  one  of  the  commonest  muta- 
tions in  melanog aster  and  was  the  first  to  be  picked  out 
in  funebris.  So  many  peculiarities  in  common  make  it 
hard  to  believe  that  they  do  not  represent  the  same  genetic 
change.  Another  mutant  also  found  in  D.  funebris  that 
parallels  one  in  D.  melanog  aster  is  called  hairless,  produc- 
ing several  similar  effects  in  both.  In  both  the  factor  is 
an  autosomal  dominant;  it  affects  the  hairs,  certain 
bristles,  and  the  second,  fourth  and  fifth  veins  of  the  wings, 
and  has  a  recessive  lethal  effect. 

One  of  the  most  interesting  ideas  that  De  Vries  brought 
forward  in  his  mutation  theory  is  that  groups  of  ''small 
species''  or  of  varieties  are  made  up  of  many  common 
genes  and  differ  in  a  relatively  small  number  of  genes. 
The  genetic  analysis  of  a  group  of  smaller  species  would 
consist  in  finding  out  how  the  different  genes  are  dis- 
tributed amongst  the  members  of  this  group.  Phylogene- 
tic  relationship  comes  to  have  a  different  significance 
from  the  traditional  relationship  expressed  in  the  descent 
theory;  but  this  point  of  view  is  so  novel  that  it  has  not 
yet  received  the  recognition  which  we  may  expect  that 
it  will  obtain  in  the  future  when  relationship  by  common 
descent  will  be  recognized  as  of  minor  importance  as 
compared  with  relationship  due  to  a  community  of  genes. 


18 


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286  PHYSICAL  BASIS  OF  HEREDITY 

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\  I 


i  I 


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292  PHYSICAL  BASIS  OF  HEEEDITY 

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296  PHYSICAL  BASIS  OF  HEEEDITY 

Sturtevant,  a.  H.  :  Linkage  in  the  Silkworm  Moth.  Am.  Nat.,  1914, 
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Sturtevant^  a.  H.  :  No  Crossing  Over  in  the  Female  of  the  Silkworm 
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Tanaka,  Y.  :  Further  Data  on  the  Reduplication  in  Silkworms.     Ibid., 

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Trow,  A.  H. :  On  the  Inheritance  of  Certain  Characters  in  the  Common 
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1913,  ii. 

Trow,  A.  H. :  Forms  of  Reduplication — Primary  and  Secondary.  Jour. 

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De  Vries,  H.  :  Ueber  die  Zwillingsbastarde  von  CEnothera  nanella.    Ber. 

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Bot.  Gaz.,  1914,  xvii. 
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Samen  durch  Druck.     Biol.  Centr. ^  1915,  xxxv. 
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INDEX 


Abnormal  abdomen,  28-29,  32,  33 
Abraxas,  175-178,  192,  248 

type,  173-177,  180 
Albinism,  67 
Albinos,  248 
Allelomorphs,  23,  59,  60 
Allen,  152 
Altenburg,  85,  146 
Amphibians,   114 

Ancyracanthus  cystidicola,  39-44 
Andalusian  fowls,  26,  32 
Androgenetic,   189 
Annelids,  114 
Antheridia,    152 
Antirrhinum,  221 
Aphids,  197,  207 
Aphid,  bearberry,  184 
Apotettix,  146 
Aphis  avenee,  208 
Archegonia,  152 
Ascaris,  51,  52,  100,  160 

nigrovenosa,  196 
Assortment,  73-79 
Atavistic  type,  252  253 

Baltzer,    215-217 

Banta,  194 

Bar-eyed,  31,  120,  121,  250 

Bataillon,  189 

Bates,  245 

Bateson,  25,  70,  85,  115-117 

Batracoseps,  46-49,  100,  113 

Baur,  85,  135,  220-222,  250 

Beaded  wing,  257-260 

Bean,  Florida  velvet,  255 

Lyon,  255 
Beans,  204 
Beafberry  aphid,  184 
Bee,  180,  181,  197,  198 
Belling,  255 
Bergson,  267 
Bifid  wing,   119 
Bion,  268 


Biophor,  234 

Bird,  174 

Biston,  53,  164 

Black  fly,  30,  31,  63,  81-83,  87-90,  96, 

123,  124,  139-144 
Bonnet,  234,  235 
Boveri,    51,   52,    160,    213-215,   223, 

224,  231 
Braehet,  189 
Bridges,  55,  97,  114,  122,  127,  129, 

138,   157,  159,  191,  200,  246 
Bursa  pastoris,  71 

Carothers,  74-77 

Castle,  86,  87,  131,  132,  146,  256 

Castration,  244 

Chamberlain,    245 

Chromosomes,  39-58,  73,  96-117 

Circotettix,  74 

Cladocerans,  186 

Clarke,  W.  T.,  210 

Clausen,  233 

Cobs  of  com,  249 

Color  blindness,  170 

Confluent  wing,  272 

Conjugation,  49,  50 

Conklin,  224,  225,  227     . 

Contamination,  34 

Corn,  85,   135,  229,   249,  252 

Correns,  219-220,  230 

Criss-cross  inheritance,  176 

Crossing  over,  87-95,  96-117,  139 

Cu^not,  25,  256 

Curved  wing,  96,  124,  140-144 

Cut  wing,  122,  248 

Ctenolabrus,  54,  232 

Cynthia,  227 

Cytoplasmic  inheritance,   219-226 

Dachs,  123 

deficiency,  124 
Daphnians,  197 
Darwin,  234,  267,  269 

301 


302 


INDEX 


Davenport,  33,  34 

Deficiency,  159 

Delage,  188 

Dichete,  261,  262 

Difflugia  corona,  207 

Digby,  151 

Dilina,  164 

Diluting  factor,  70 

Diploid,  84,   153-154 

Disjunction,  23 

Doncaster,  55,  164,   177,  192,  193 

Dominance,  25,  60 

Double  crossing  over,   119 

Driesch,  231,  242 

Drosera  longifolia,  160 
rotundifolia,  160 

Drosophila  busckii,  57,  86,  135 
funebris,  273 
melanica,  57 

melanogaster,  27,  28,  30,  31,  37, 
54,  57,  63,  66,  80,  84,  85,  87, 
94-96,  113-115,  118,  127,  129, 
130,  133,  134,  139,  143,  145,  146, 
157,  159,  167,  170,  176,  177,  190, 
191,  198-200,  236-238,  240,  248, 
249,  251,  253,  256-258,  260,  263, 
270,  271 
repleta,  86,  135 
virilis,  85,   135,  272 

Dumpy  wing,  66,  67 

Duplication,  159 

East,  229,  230 
Ebony  fly,  27,  63 
Echinus  eye,   122 
Emerson,  249 
Endosperm,  229,  230 
Engledow,  85 
Enzyme,  245 
Eosin  eye,  70 
Equation  division,  43 
Euglena,   185,  187 
Ewing,  208 

Federley,  54,  162 

Flinty  corn,  229 

Flowery  corn,  229 

Fish,  54 

Forked  bristles,  92,  93,  122,  272 

Four-o'clock,  25,  220 


Fowls,  239,  270 
Fruit  fly,  252,  272 
Fundulus,  54,  232 

Game  bantam,  244 

Gamete,  84 

Gametic  lethal,  254,  265 

Garnet  eye,  122 

Gates,  149,  155,  156 

Geerts,  155 

Gene,  234-246 

Genes,  the  order  of,  118-125 

Germ-plasm,  234,  239 

Gigas,  Oenothera,  149,  265 

Goldschmidt,  245 

Goodale,    86,    180,    244 

Goodspeed,  233,  256 

Gowen,  127,  146 

Grasshopper,  74,  75 

Gregory,  85,  146,  150 

Groundsel,  85 

Grouse  locust,  146 

Guinea  pigs,  albino,  271 

Guyer,  136,  137,  170,  174,  178,  179 

Gynandromorph,    190-193 

Gypsy  moth,   194 

Haemophilia,   170 
Hairless  fly,  273 
Hance,  157 
Haploid,  55,  153,  154 
Harrison,  55,  164 
Hayes,  229,  230 
Hegner,  207 
Herbst,  213,  217,  218 
Herlandt,  188 
Hermaphrodite,  197 

plant,  154 
Hertwig,  G.,  188 
Hertwig,  O.,  114,  188 
Hertwig,  R.,  188  ' 

Heterozygous,  23 
Homozygous,  23 
Hornet,  181 
Hydatina  senta,  185 

Individuality    of    the    chromosomes, 

51 
Insects,  114 
Interference,  126-132 


i 


I 


INDEX 


303 


Internal  secretion,  244 
Intersexes,   193 

Janssens,   46,   48,    102,    110,   112 
Jennings,  207 
Jesenko,  256 
Johannsen,  204-206,  246 
Jones,  85 
Keeble,  150 
King,  231 

Kusohakewitsch,    188 
Kuttner,  O.,  194 
Lamarckian  theory,  267 
Langshan,  177-180 
Leptotene  thread,  100,  107 
Lethal,  254,  257-265 

factor,  198-200 
Linkage,  80-86,  94 

groups,  133 
Lippincott,  '26 
Little,  256 
Liverwort,  152 

Loeb,   J.,   189,  225,  226,  231,  245 
Lutz,  A.,  155,  157 
Lymantria  dispar,  194 

japonica,  194 

Maize,  229 

Man,  137,  170,  200 

McClung,  165 

Marchal,  Elie  and  fimile,  151-154 

Marechal,  49 

Marshall,  35 

Maternal  inheritance,  227-233 

Maturation  division,  43 

May,  250 

Melandrium,  221 

Melanic  form®,  248,  271 

Men,  136 

Mendel,  15-17,  19,  22,  23,  37,  38,  85, 

236 
Mendelism,  36 
Mendel's  first  law,  19-38,  73 

second  law,  59-72,  79 
Menidia,  232 
Metapodius,  57 
Metz,  57,  85,    135,   137,  272 
Meves,  181,  182 
Mice,  albino,  271 
Miniature  wing,  66,  67,  91-93 


Mirabilis  jalapa,  25,  32,  219,  220 

Mitotic  division,  40 

Modifying  genes,  246 

Moenkhaus,   55 

Moore,  231 

Morgan,  35,  112,  191 

Morris,  55 

Moss,  151-154 

Moths,  55 

Mouse,  67,  68,  70,  135,   137 

albino,   70 

blue,  70 

chocolate,  70 

silver-fawn,  70 

yellow,  257 
Muller,   35,  85,    127,   129,   130,   150, 

258,  260 
Mulsow,  39,  41 

Multiple    Allelomorphs,    251-254 
Mutation,  247-273 

Nabours  37,  86,  146 
Natural  selection,  267-270 
Nicotiana  sylvestris,  256 

tabacum,  256 
Non-disjunction,  200-203 
Notch,  35 
Notch  wing,  248 
Nova  Scotia  stock,   143,    144 

Oats,  85,   135 
Oenothera  laeta,  264 

Lamarckiana,    53,    85,    149,    155, 
262-264 

Bcintillans,  157 

velutina,  264 
Ouslow,  245 
Oogonia,    154 
Orthogenesis,  266 
Osawa,  256 

Pachytene  thread,  49 
Packard,  188,  189 
Paleontologists,   266 
Parthenogenesis,  39,  180,  204-211 

artificial,  188,  189 
Pea  comb,  68,  69 
Pea,  Garden,  19-23,  32,  59-62,  85 

Palestine,  256 

wild,  270 


304 


INDEX 


Peach  eye,  261 
Pelargonium,    221 
Petrunkewitch,  198 
Phaseolua  vulgaris,  204 
Phillips,  207 
Phrynotettix,   104-110 
Phylloxera  caryoecaulis,  183 
Phylloxerans,  197 
Pinney,  55,  232 
Pisum  humile,  256 
sativum,  19,  134 
Plough,   96-99,    115,    139-142 
Plymouth  rock,  barred,  176,  180 
Polar   body,  40 
Polytoma,  186 
Prematuration  stage,  142 
Primary  split,  101,  102,   107,   108 
Primrose,  85 
Primula  floribunda,  150,  151 

kewensis,   151 

sinensis,  85,  87,   135,  146,   150 

verticillata,   150,   151 
Pristiurus  melanostomus,  49 
Protenor,  55,  56 
Protozoa,  207 

Punnett,  70,  85,  115-117,  250 
Pure  culture,  250 
lines,  204,  211 
Purple  eye,  96,  124,  139-144,  240 
Pygaera,  161,  163 

Rabbits,  albino,  271 
Rat,  black,  271 

Norway,  271 

roof,  271 

yellow,  271,  272 
Recessive,  23,  25 
Reduction  division,  43,  75,  101 
Reduplication,  115-117 
Regeneration,   154 
Reversal  of  dominance,  33 
Riddle,  194,  196,  245 
Ring  dove,  194 

Robertson,  74,    102,    103,   111-112 
Rose  comb,  68,  69 
Rosenberg,  160,   161,  256 
Rotifer,  185,  197,  198 
Roux,  142 
Rudimentary   wing,   248 


Sable  body  color,  158,  159 
Saunders,  85,  254 
Satsuma,  256 
Schreiners,  44 
Scute,  122 
Sebrights,  243,  244 
Secondary  split,  101,  102 
Sea  urchin,  188,  189 
Segregation,   16,  23,  39 
Seller,  174,   179 
Selachian  egg,  50 
Selachians,   114 
Senecio  vulgaris,  85 
Sex,  196-203 

chromosomes,  42,  57 

determination,  180 

genes,  193 

linked  characters,  84 

ratios,    197-200 
Shinji,  210  ^ 

Shull,   71 

Silkworm,  135,  137,  146,  186,  228 
Simocephalus,    194 
Smerinthus,  164 
Single  comb,  68,  69 
Snapdragon,  85,   135,  255 
Sooty  fly,  27 
Species,  270 
Speck,  123,  144 
Spencer,  Herbert,  234 
Spermatogonia,  154 
Spermatogenesis,  104-110 
Spermatozoon,  43 
Sphserechinus,  216,  217 
Spores,  151 
Sports,  248 
Squirrels,  albino,  271 
Star  eye,  122,  123 
Stark,  256 
Stenotomus,  232 
Stevens,  145,  165 
Stocks,   85,  254 
Stomps,  149,  155 
Streptopelia  abla,  194 
Strong,    196 
Strongylocentrotus,   216,   217 

Franciscanus,  231,   232 

purpureus,  231,  232 


INDEX 


305 


Sturtevant,   86,    114,   117,   125,    127, 

143-145,  191,  260,  261,  272 
Sundew,  256 
Surface,  85,  135 
Sutton,  16,  256 
Sweet  peas,  70,  85 
Synaptic  stage,  47 
Syndactyls,  33,  34 
Synizesis  stage,  46 

Tadpole,  189 

Tanaka,  86,  135 

Temperature,  96-97,   139-142 

Tennent,  188,  231 

Tetrads,   39,  43,  50,  75,   107,   114 

Tetraploid,    150,   154,    159 

Tettigidea,  111 

Tomatoes,  85 

Tomopteris,  44-46,  100,  113 

Toyama,  191,  192,  228 

Trimerotropsis,  74,  75,  78 

Triploid,  155,  160 

Trow,  117 

Truncate  wing,  248 

Turtle  dove,  194 

Turtur  orientalis,  194 

Twin  hybrids,  264 

Variation — in  linkage,  139-146 
Variation — in    number    of    chromo- 
somes, 147-164 
Vermilion  eye,  158,  159,  248 
Vestigial  fly,  63,  81-83,  87-90,  123, 
124 


Vilmorin,  85 

Vinegar  fly,  27,  28,  70,  168-170,  172 
Voinov,  74 

Vries,    de,    85,    149,    155,    263,    264, 
266,  273 

Walnut  comb,  68,  69 

Warren,  85 

Wasps,  198 

Weatherwax,  229 

Weinstein,  127,  129,  131 

Weismann,  234,  235,  242 

Wenrich,  74,  102,  110,  113 

Wheat,  85,  135 

White,  O.  E.,  20,  67,  85,  134 

White-eyed  fly,  91-93,  118,  167,  168, 

171-173,  238,  248,  271 
Whiting,  70 
Whitney,  185,  186 
Wilson,  E.  B.,  55,  165 
Winiwarter,  136-137,  170 
Wright,  86,  87 

Xenia,  230 

Yatsu,  135,   137 

Yellow  mouse,  257 

Yellow- winged  fly,  91,  118,  120,  171- 

173,  190,  250,  272 
Yocom,  137 

Zea  mais,  229,  249 
Zygote,  84 
Zygotene  stage,  107 
Zygotic    lethal,    254,    256 


nOFERTY  LIBURY 

N.  C.  State  College 


HECKMAN 

BINDERY  INC. 

FEB  85 


N.  MANCHESTER, 
INDIANA  46962 


J 


